NAME
perlfaq7 - General Perl Language Issues ($Revision: 1.18 $, $Date:
2004/11/03 22:54:08 $)
DESCRIPTION
This section deals with general Perl language issues that don't clearly fit into any of the other sections. CCaann II ggeett aa BBNNFF//yyaacccc//RREE ffoorr tthhee PPeerrll llaanngguuaaggee?? There is no BNF, but you can paw your way through the yacc grammar in perly.y in the source distribution if you're particularly brave. The grammar relies on very smart tokenizing code, so be prepared to venture into toke.c as well. In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF. The work of parsing perl is distributed between yacc, the lexer, smoke and mirrors."WWhhaatt aarree aallll tthheessee $$@@%%&&** ppuunnccttuuaattiioonn ssiiggnnss,, aanndd hhooww ddoo II kknnooww wwhheenn ttoo
uussee tthheemm?? They are type specifiers, as detailed in perldata:$ for scalar values (number, string or reference)
@ for arrays% for hashes (associative arrays)
& for subroutines (aka functions, procedures, methods) * for all types of that symbol name. In version 4 you used them like pointers, but in modern perls you can just use references. There are couple of other symbols that you're likely to encounter that aren't really type specifiers: <> are used for inputting a record from a filehandle. \ takes a reference to something. Note thatis neither the type specifier for files nor the name of the handle. It is the "<>" operator applied to the handle FILE. It reads one line (well, record-see "$/" in perlvar) from the handle FILE
in scalar context, or all lines in list context. When performing open,close, or any other operation besides "<>" on files, or even when talk-
ing about the handle, do not use the brackets. These are correct: "eof(FH)", "seek(FH, 0, 2)" and "copying from STDIN to FILE".DDoo II aallwwaayyss//nneevveerr hhaavvee ttoo qquuoottee mmyy ssttrriinnggss oorr uussee sseemmiiccoolloonnss aanndd ccoomm-
mmaass??Normally, a bareword doesn't need to be quoted, but in most cases prob-
ably should be (and must be under "use strict"). But a hash key con-
sisting of a simple word (that isn't the name of a defined subroutine)and the left-hand operand to the "=>" operator both count as though
they were quoted: This is like this------ --------
$foo{line} $foo{"line"}
bar => stuff "bar" => stuff The final semicolon in a block is optional, as is the final comma in a list. Good style (see perlstyle) says to put them in except forone-liners:
if ($whoops) { exit 1 }
@nums = (1, 2, 3);if ($whoops) {
exit 1; } @lines = ( "There Beren came from mountains cold", "And lost he wandered under leaves", ); HHooww ddoo II sskkiipp ssoommee rreettuurrnn vvaalluueess?? One way is to treat the return values as a list and index into it:$dir = (getpwnam($user))[7];
Another way is to use undef as an element on the left-hand-side:
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
You can also use a list slice to select only the elements that you need:($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];
HHooww ddoo II tteemmppoorraarriillyy bblloocckk wwaarrnniinnggss?? If you are running Perl 5.6.0 or better, the "use warnings" pragma allows fine control of what warning are produced. See perllexwarn for more details. {no warnings; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
} Additionally, you can enable and disable categories of warnings. You turn off the categories you want to ignore and you can still get other categories of warnings. See perllexwarn for the complete details, including the category names and hierarchy. { no warnings 'uninitialized';$a = $b + $c;
}If you have an older version of Perl, the $^W variable (documented in
perlvar) controls runtime warnings for a block: {local $^W = 0; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
} Note that like all the punctuation variables, you cannot currently usemy() on $^W, only local().
WWhhaatt''ss aann eexxtteennssiioonn?? An extension is a way of calling compiled C code from Perl. Reading perlxstut is a good place to learn more about extensions. WWhhyy ddoo PPeerrll ooppeerraattoorrss hhaavvee ddiiffffeerreenntt pprreecceeddeennccee tthhaann CC ooppeerraattoorrss?? Actually, they don't. All C operators that Perl copies have the same precedence in Perl as they do in C. The problem is with operators thatC doesn't have, especially functions that give a list context to every-
thing on their right, eg. print, chmod, exec, and so on. Such func-
tions are called "list operators" and appear as such in the precedence table in perlop. A common mistake is to write:unlink $file || die "snafu";
This gets interpreted as:unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or use the super low precedence "or" operator:(unlink $file) || die "snafu";
unlink $file or die "snafu";
The "English" operators ("and", "or", "xor", and "not") deliberatelyhave precedence lower than that of list operators for just such situa-
tions as the one above. Another operator with surprising precedence is exponentiation. Itbinds more tightly even than unary minus, making "-2**2" product a neg-
ative not a positive four. It is also right-associating, meaning that
"2**3**2" is two raised to the ninth power, not eight squared.Although it has the same precedence as in C, Perl's "?:" operator pro-
duces an lvalue. This assigns $x to either $a or $b, depending on the
trueness of $maybe:
($maybe ? $a : $b) = $x;
HHooww ddoo II ddeeccllaarree//ccrreeaattee aa ssttrruuccttuurree?? In general, you don't "declare" a structure. Just use a (probably anonymous) hash reference. See perlref and perldsc for details. Here's an example:$person = {}; # new anonymous hash
$person->{AGE} = 24; # set field AGE to 24
$person->{NAME} = "Nat"; # set field NAME to "Nat"
If you're looking for something a bit more rigorous, try perltoot. HHooww ddoo II ccrreeaattee aa mmoodduullee??A module is a package that lives in a file of the same name. For exam-
ple, the Hello::There module would live in Hello/There.pm. For details, read perlmod. You'll also find Exporter helpful. If you'rewriting a C or mixed-language module with both C and Perl, then you
should study perlxstut. The "h2xs" program will create stubs for all the important stuff for you:% h2xs -XA -n My::Module
The "-X" switch tells "h2xs" that you are not using "XS" extension
code. The "-A" switch tells "h2xs" that you are not using the
AutoLoader, and the "-n" switch specifies the name of the module. See
h2xs for more details. HHooww ddoo II ccrreeaattee aa ccllaassss?? See perltoot for an introduction to classes and objects, as well as perlobj and perlbot. HHooww ccaann II tteellll iiff aa vvaarriiaabbllee iiss ttaaiinntteedd??You can use the tainted() function of the Scalar::Util module, avail-
able from CPAN (or included with Perl since release 5.8.0). See also "Laundering and Detecting Tainted Data" in perlsec. WWhhaatt''ss aa cclloossuurree?? Closures are documented in perlref.Closure is a computer science term with a precise but hard-to-explain
meaning. Closures are implemented in Perl as anonymous subroutines with lasting references to lexical variables outside their own scopes. These lexicals magically refer to the variables that were around when the subroutine was defined (deep binding). Closures make sense in any programming language where you can have the return value of a function be itself a function, as you can in Perl.Note that some languages provide anonymous functions but are not capa-
ble of providing proper closures: the Python language, for example. For more information on closures, check out any textbook on functionalprogramming. Scheme is a language that not only supports but encour-
ages closures.Here's a classic function-generating function:
sub addfunctiongenerator { return sub { shift() + shift() }; }$addsub = addfunctiongenerator();
$sum = $addsub->(4,5); # $sum is 9 now.
The closure works as a function template with some customization slots left out to be filled later. The anonymous subroutine returned by addfunctiongenerator() isn't technically a closure because it refers to no lexicals outside its own scope. Contrast this with the following makeadder() function, in which the returned anonymous function contains a reference to a lexical variable outside the scope of that function itself. Such a reference requires that Perl return a proper closure, thus locking in for all time the value that the lexical had when the function was created. sub makeadder {my $addpiece = shift;
return sub { shift() + $addpiece };
}$f1 = makeadder(20);
$f2 = makeadder(555);
Now "&$f1($n)" is always 20 plus whatever $n you pass in, whereas
"&$f2($n)" is always 555 plus whatever $n you pass in. The $addpiece
in the closure sticks around. Closures are often used for less esoteric purposes. For example, when you want to pass in a bit of code into a function:my $line;
timeout( 30, sub { $line =
} ); If the code to execute had been passed in as a string, '$line =
', there would have been no way for the hypothetical timeout() function to access the lexical variable $line back in its caller's
scope. WWhhaatt iiss vvaarriiaabbllee ssuuiicciiddee aanndd hhooww ccaann II pprreevveenntt iitt?? Variable suicide is when you (temporarily or permanently) lose the value of a variable. It is caused by scoping through my() and local()interacting with either closures or aliased foreach() iterator vari-
ables and subroutine arguments. It used to be easy to inadvertently lose a variable's value this way, but now it's much harder. Take this code:my $f = "foo";
sub T {while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
} T;print "Finally $f\n";
The $f that has "bar" added to it three times should be a new $f ("my
$f" should create a new local variable each time through the loop). It
isn't, however. This was a bug, now fixed in the latest releases (tested against 5.00405, 5.00503, and 5.00556). How can I pass/return a {Function, FileHandle, Array, Hash, Method, RReeggeexx}}?? With the exception of regexes, you need to pass references to theseobjects. See "Pass by Reference" in perlsub for this particular ques-
tion, and perlref for information on references. See ``Passing Regexes'', below, for information on passing regular expressions. Passing Variables and Functions Regular variables and functions are quite easy to pass: just pass in a reference to an existing or anonymous variable or function:func( \$somescalar );
func( \@somearray ); func( [ 1 .. 10 ] );func( \%somehash );
func( { this => 10, that => 20 } ); func( \&somefunc );func( sub { $[0] ** $[1] } );
Passing Filehandles As of Perl 5.6, you can represent filehandles with scalar variables which you treat as any other scalar.open my $fh, $filename or die "Cannot open $filename! $!";
func( $fh );
sub func {my $passedfh = shift;
my $line = <$fh>;
} Before Perl 5.6, you had to use the *FH or "\*FH" notations. Theseare "typeglobs"-see "Typeglobs and Filehandles" in perldata and
especially "Pass by Reference" in perlsub for more information. Passing Regexes To pass regexes around, you'll need to be using a release of Perl sufficiently recent as to support the "qr//" construct, pass aroundstrings and use an exception-trapping eval, or else be very, very
clever. Here's an example of how to pass in a string to be regex compared using "qr//":sub compare($$) {
my ($val1, $regex) = @;
my $retval = $val1 =~ /$regex/;
return $retval;
}$match = compare("old McDonald", qr/d.*D/i);
Notice how "qr//" allows flags at the end. That pattern was com-
piled at compile time, although it was executed later. The nifty "qr//" notation wasn't introduced until the 5.005 release. Before that, you had to approach this problem much less intuitively. For example, here it is again if you don't have "qr//":sub compare($$) {
my ($val1, $regex) = @;
my $retval = eval { $val1 =~ /$regex/ };
die if $@;
return $retval;
}$match = compare("old McDonald", q/($?i)d.*D/);
Make sure you never say something like this:return eval "\$val =~ /$regex/"; # WRONG
or someone can sneak shell escapes into the regex due to the doubleinterpolation of the eval and the double-quoted string. For exam-
ple:$patternofevil = 'danger ${ system("rm -rf * &") } danger';
eval "\$string =~ /$patternofevil/";
Those preferring to be very, very clever might see the O'Reilly book, Mastering Regular Expressions, by Jeffrey Friedl. Page 273's BuildMatchManyFunction() is particularly interesting. A complete citation of this book is given in perlfaq2. Passing Methods To pass an object method into a subroutine, you can do this:callalot(10, $someobj, "methname")
sub callalot {my ($count, $widget, $trick) = @;
for (my $i = 0; $i < $count; $i++) {
$widget->$trick();
} } Or, you can use a closure to bundle up the object, its method call, and arguments:my $whatnot = sub { $someobj->obfuscate(@args) };
func($whatnot);
sub func {my $code = shift;
&$code();
} You could also investigate the can() method in the UNIVERSAL class (part of the standard perl distribution). HHooww ddoo II ccrreeaattee aa ssttaattiicc vvaarriiaabbllee?? As with most things in Perl, TMTOWTDI. What is a "static variable" inother languages could be either a function-private variable (visible
only within a single function, retaining its value between calls tothat function), or a file-private variable (visible only to functions
within the file it was declared in) in Perl.Here's code to implement a function-private variable:
BEGIN {my $counter = 42;
sub prevcounter { return -$counter }
sub nextcounter { return $counter++ }
}Now prevcounter() and nextcounter() share a private variable $counter
that was initialized at compile time.To declare a file-private variable, you'll still use a my(), putting
the declaration at the outer scope level at the top of the file. Assume this is in file Pax.pm: package Pax;my $started = scalar(localtime(time()));
sub begun { return $started }
When "use Pax" or "require Pax" loads this module, the variable will beinitialized. It won't get garbage-collected the way most variables
going out of scope do, because the begun() function cares about it, butno one else can get it. It is not called $Pax::started because its
scope is unrelated to the package. It's scoped to the file. You could conceivably have several packages in that same file all accessing the same private variable, but another file with the same package couldn't get to it. See "Persistent Private Variables" in perlsub for details. WWhhaatt''ss tthhee ddiiffffeerreennccee bbeettwweeeenn ddyynnaammiicc aanndd lleexxiiccaall ((ssttaattiicc)) ssccooppiinngg?? BBeettwweeeenn local() aanndd my()??"local($x)" saves away the old value of the global variable $x and
assigns a new value for the duration of the subroutine which is visible in other functions called from that subroutine. This is done atrun-time, so is called dynamic scoping. local() always affects global
variables, also called package variables or dynamic variables."my($x)" creates a new variable that is only visible in the current
subroutine. This is done at compile-time, so it is called lexical or
static scoping. my() always affects private variables, also called lexical variables or (improperly) static(ly scoped) variables. For instance: sub visible {print "var has value $var\n";
} sub dynamic {local $var = 'local'; # new temporary value for the still-global
visible(); # variable called $var
} sub lexical {my $var = 'private'; # new private variable, $var
visible(); # (invisible outside of sub scope)
}$var = 'global';
visible(); # prints global
dynamic(); # prints local
lexical(); # prints global
Notice how at no point does the value "private" get printed. That'sbecause $var only has that value within the block of the lexical()
function, and it is hidden from called subroutine. In summary, local() doesn't make what you think of as private, local variables. It gives a global variable a temporary value. my() is what you're looking for if you want private variables. See "Private Variables via my()" in perlsub and "Temporary Values via local()" in perlsub for excruciating details. HHooww ccaann II aacccceessss aa ddyynnaammiicc vvaarriiaabbllee wwhhiillee aa ssiimmiillaarrllyy nnaammeedd lleexxiiccaall iiss iinn ssccooppee?? If you know your package, you can just mention it explicitly, as in$SomePack::var. Note that the notation $::var is nnoott the dynamic $var
in the current package, but rather the one in the "main" package, asthough you had written $main::var.
use vars '$var';
local $var = "global";
my $var = "lexical";
print "lexical is $var\n";
print "global is $main::var\n";
Alternatively you can use the compiler directive our() to bring a dynamic variable into the current lexical scope.require 5.006; # our() did not exist before 5.6
use vars '$var';
local $var = "global";
my $var = "lexical";
print "lexical is $var\n";
{our $var;
print "global is $var\n";
} WWhhaatt''ss tthhee ddiiffffeerreennccee bbeettwweeeenn ddeeeepp aanndd sshhaallllooww bbiinnddiinngg?? In deep binding, lexical variables mentioned in anonymous subroutines are the same ones that were in scope when the subroutine was created. In shallow binding, they are whichever variables with the same names happen to be in scope when the subroutine is called. Perl always uses deep binding of lexical variables (i.e., those created with my()). However, dynamic variables (aka global, local, or package variables) are effectively shallowly bound. Consider this just one more reason not to use them. See the answer to "What's a closure?".WWhhyy ddooeessnn''tt ""mmyy(($$ffoooo)) == <
"my()" and "local()" give list context to the right hand side of "=".>;;"" wwoorrkk rriigghhtt?? The
read operation, like so many of Perl's functions and opera- tors, can tell which context it was called in and behaves appropri-
ately. In general, the scalar() function can help. This function does nothing to the data itself (contrary to popular myth) but rather tells its argument to behave in whatever its scalar fashion is. If that function doesn't have a defined scalar behavior, this of course doesn't help you (such as with sort()). To enforce scalar context in this particular case, however, you need merely omit the parentheses:local($foo) =
; # WRONG local($foo) = scalar(
); # ok local $foo =
You should probably be using lexical variables anyway, although the issue is the same here:; # right my($foo) =
; # WRONG my $foo =
HHooww ddoo II rreeddeeffiinnee aa bbuuiillttiinn ffuunnccttiioonn,, ooppeerraattoorr,, oorr mmeetthhoodd??; # right Why do you want to do that? :-)
If you want to override a predefined function, such as open(), then you'll have to import the new definition from a different module. See"Overriding Built-in Functions" in perlsub. There's also an example in
"Class::Template" in perltoot. If you want to overload a Perl operator, such as "+" or "**", then you'll want to use the "use overload" pragma, documented in overload. If you're talking about obscuring method calls in parent classes, see "Overridden Methods" in perltoot. WWhhaatt''ss tthhee ddiiffffeerreennccee bbeettwweeeenn ccaalllliinngg aa ffuunnccttiioonn aass &&ffoooo aanndd foo()?? When you call a function as &foo, you allow that function access to your current @ values, and you bypass prototypes. The functiondoesn't get an empty @-it gets yours! While not strictly speaking a
bug (it's documented that way in perlsub), it would be hard to consider this a feature in most cases. When you call your function as "&foo()", then you do get a new @, but prototyping is still circumvented. Normally, you want to call a function using "foo()". You may only omit the parentheses if the function is already known to the compiler because it already saw the definition ("use" but not "require"), or via a forward reference or "use subs" declaration. Even in this case, you get a clean @ without any of the old values leaking through where they don't belong. HHooww ddoo II ccrreeaattee aa sswwiittcchh oorr ccaassee ssttaatteemmeenntt?? This is explained in more depth in the perlsyn. Briefly, there's no official case statement, because of the variety of tests possible in Perl (numeric comparison, string comparison, glob comparison, regex matching, overloaded comparisons, ...). Larry couldn't decide how best to do this, so he left it out, even though it's been on the wish list since perl1. Starting from Perl 5.8 to get switch and case one can use the Switch extension and say: use Switch; after which one has switch and case. It is not as fast as it could be because it's not really part of the language (it's done using source filters) but it is available, and it's very flexible. But if one wants to use pure Perl, the general answer is to write a construct like this:for ($variabletotest) {
if (/pat1/) { } # do something
elsif (/pat2/) { } # do something else
elsif (/pat3/) { } # do something else
else { } # default
} Here's a simple example of a switch based on pattern matching, this time lined up in a way to make it look more like a switch statement. We'll do a multiway conditional based on the type of reference storedin $whatchamacallit:
SWITCH: for (ref $whatchamacallit) {
/^$/ && die "not a reference";
/SCALAR/ && do {printscalar($$ref);
last SWITCH; }; /ARRAY/ && do {printarray(@$ref);
last SWITCH; }; /HASH/ && do {printhash(%$ref);
last SWITCH; }; /CODE/ && do { warn "can't print function ref"; last SWITCH; };# DEFAULT
warn "User defined type skipped"; }See "perlsyn/"Basic BLOCKs and Switch Statements"" for many other exam-
ples in this style.Sometimes you should change the positions of the constant and the vari-
able. For example, let's say you wanted to test which of many answersyou were given, but in a case-insensitive way that also allows abbrevi-
ations. You can use the following technique if the strings all start with different characters or if you want to arrange the matches so that one takes precedence over another, as "SEND" has precedence over "STOP" here:chomp($answer = <>);
if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
A totally different approach is to create a hash of function refer-
ences.my %commands = (
"happy" => \&joy, "sad", => \&sullen, "done" => sub { die "See ya!" }, "mad" => \&angry, ); print "How are you? ";chomp($string =
); if ($commands{$string}) {
$commands{$string}->();
} else {print "No such command: $string\n";
} HHooww ccaann II ccaattcchh aacccceesssseess ttoo uunnddeeffiinneedd vvaarriiaabblleess,, ffuunnccttiioonnss,, oorr mmeetthhooddss?? The AUTOLOAD method, discussed in "Autoloading" in perlsub and"AUTOLOAD: Proxy Methods" in perltoot, lets you capture calls to unde-
fined functions and methods. When it comes to undefined variables that would trigger a warning under "use warnings", you can promote the warning to an error. use warnings FATAL => qw(uninitialized); WWhhyy ccaann''tt aa mmeetthhoodd iinncclluuddeedd iinn tthhiiss ssaammee ffiillee bbee ffoouunndd?? Some possible reasons: your inheritance is getting confused, you've misspelled the method name, or the object is of the wrong type. Check out perltoot for details about any of the above cases. You may alsouse "print ref($object)" to find out the class $object was blessed
into.Another possible reason for problems is because you've used the indi-
rect object syntax (eg, "find Guru "Samy"") on a class name before Perl has seen that such a package exists. It's wisest to make sure your packages are all defined before you start using them, which will be taken care of if you use the "use" statement instead of "require". Ifnot, make sure to use arrow notation (eg., "Guru->find("Samy")")
instead. Object notation is explained in perlobj. Make sure to read about creating modules in perlmod and the perils of indirect objects in "Method Invocation" in perlobj. HHooww ccaann II ffiinndd oouutt mmyy ccuurrrreenntt ppaacckkaaggee?? If you're just a random program, you can do this to find out what the currently compiled package is:my $packname = PACKAGE;
But, if you're a method and you want to print an error message thatincludes the kind of object you were called on (which is not necessar-
ily the same as the one in which you were compiled): sub amethod {my $self = shift;
my $class = ref($self) || $self;
warn "called me from a $class object";
} HHooww ccaann II ccoommmmeenntt oouutt aa llaarrggee bblloocckk ooff ppeerrll ccooddee?? You can use embedded POD to discard it. Enclose the blocks you want to comment out in POD markers, for example "=for nobody" and "=cut" (which marks ends of POD blocks).# program is here
=for nobody all of this stuff here will be ignored by everyone =cut# program continues
The pod directives cannot go just anywhere. You must put a pod direc-
tive where the parser is expecting a new statement, not just in the middle of an expression or some other arbitrary grammar production. See perlpod for more details. HHooww ddoo II cclleeaarr aa ppaacckkaaggee??Use this code, provided by Mark-Jason Dominus:
sub scrubpackage { no strict 'refs';my $pack = shift;
die "Shouldn't delete main package"if $pack eq "" || $pack eq "main";
my $stash = *{$pack . '::'}{HASH};
my $name;
foreach $name (keys %$stash) {
my $fullname = $pack . '::' . $name;
# Get rid of everything with that name.
undef $$fullname;
undef @$fullname;
undef %$fullname;
undef &$fullname;
undef *$fullname;
} }Or, if you're using a recent release of Perl, you can just use the Sym-
bol::deletepackage() function instead. HHooww ccaann II uussee aa vvaarriiaabbllee aass aa vvaarriiaabbllee nnaammee?? Beginners often think they want to have a variable contain the name of a variable.$fred = 23;
$varname = "fred";
++$$varname; # $fred now 24
This works sometimes, but it is a very bad idea for two reasons. The first reason is that this technique only works on global variables.That means that if $fred is a lexical variable created with my() in the
above example, the code wouldn't work at all: you'd accidentally access the global and skip right over the private lexical altogether. Global variables are bad because they can easily collide accidentally and ingeneral make for non-scalable and confusing code.
Symbolic references are forbidden under the "use strict" pragma. They are not true references and consequently are not reference counted or garbage collected.The other reason why using a variable to hold the name of another vari-
able is a bad idea is that the question often stems from a lack of understanding of Perl data structures, particularly hashes. By usingsymbolic references, you are just using the package's symbol-table hash
(like %main::) instead of a user-defined hash. The solution is to use
your own hash or a real reference instead.$USERVARS{"fred"} = 23;
$varname = "fred";
$USERVARS{$varname}++; # not $$varname++
There we're using the %USERVARS hash instead of symbolic references.
Sometimes this comes up in reading strings from the user with variablereferences and wanting to expand them to the values of your perl pro-
gram's variables. This is also a bad idea because it conflates theprogram-addressable namespace and the user-addressable one. Instead of
reading a string and expanding it to the actual contents of your pro-
gram's own variables:$str = 'this has a $fred and $barney in it';
$str =~ s/(\$\w+)/$1/eeg; # need double eval
it would be better to keep a hash around like %USERVARS and have vari-
able references actually refer to entries in that hash:$str =~ s/\$(\w+)/$USERVARS{$1}/g; # no /e here at all
That's faster, cleaner, and safer than the previous approach. Of course, you don't need to use a dollar sign. You could use your own scheme to make it less confusing, like bracketed percent symbols, etc.$str = 'this has a %fred% and %barney% in it';
$str =~ s/%(\w+)%/$USERVARS{$1}/g; # no /e here at all
Another reason that folks sometimes think they want a variable to con-
tain the name of a variable is because they don't know how to build proper data structures using hashes. For example, let's say theywanted two hashes in their program: %fred and %barney, and that they
wanted to use another scalar variable to refer to those by name.$name = "fred";
$$name{WIFE} = "wilma"; # set %fred
$name = "barney";
$$name{WIFE} = "betty"; # set %barney
This is still a symbolic reference, and is still saddled with the prob-
lems enumerated above. It would be far better to write:$folks{"fred"}{WIFE} = "wilma";
$folks{"barney"}{WIFE} = "betty";
And just use a multilevel hash to start with. The only times that you absolutely must use symbolic references are when you really must refer to the symbol table. This may be because it's something that can't take a real reference to, such as a format name. Doing so may also be important for method calls, since these always go through the symbol table for resolution. In those cases, you would turn off "strict 'refs'" temporarily so you can play around with the symbol table. For example: @colors = qw(red blue green yellow orange purple violet);for my $name (@colors) {
no strict 'refs'; # renege for the block
*$name = sub { "@" };
}All those functions (red(), blue(), green(), etc.) appear to be sepa-
rate, but the real code in the closure actually was compiled only once. So, sometimes you might want to use symbolic references to directly manipulate the symbol table. This doesn't matter for formats, handles,and subroutines, because they are always global-you can't use my() on
them. For scalars, arrays, and hashes, though-and usually for subrou-
tines- you probably only want to use hard references.
WWhhaatt ddooeess ""bbaadd iinntteerrpprreetteerr"" mmeeaann?? The "bad interpreter" message comes from the shell, not perl. The actual message may vary depending on your platform, shell, and locale settings.If you see "bad interpreter - no such file or directory", the first
line in your perl script (the "shebang" line) does not contain the right path to perl (or any other program capable of running scripts). Sometimes this happens when you move the script from one machine toanother and each machine has a different path to perl--/usr/bin/perl
versus /usr/local/bin/perl for instance. If you see "bad interpreter: Permission denied", you need to make your script executable. In either case, you should still be able to run the scripts with perl explicitly:% perl script.pl
If you get a message like "perl: command not found", perl is not in your PATH, which might also mean that the location of perl is not where you expect it so you need to adjust your shebang line. AUTHOR AND COPYRIGHTCopyright (c) 1997-2002 Tom Christiansen and Nathan Torkington. All
rights reserved. This documentation is free; you can redistribute it and/or modify it under the same terms as Perl itself. Irrespective of its distribution, all code examples in this file are hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit would be courteous but is not required.perl v5.8.6 2004-11-05 PERLFAQ7(1)