use, BEGIN and what's Perl doing with them

Nov 24

von Sebastian am 24.11.2011 um 6:57 in English, Perl

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use-lines and BEGIN-blocks are not processed like they appear within a file.

Let's try out this sample script:

#!/usr/bin/perluse Mod::One;BEGIN {    use Mod::Two;    print 1;    use Mod::Three;}END {    print 7;}use Mod::Four;print 2;use Mod::Five;
print 3;
a();
print 6;
sub a {    print 4;    use Mod::Six;    print 5;}

This file isn't processed in the order of it's lines. Actually processing is done this way:

#!/usr/bin/perl# use-lines and BEGIN-blocks are processed in order, but everything else is ignored:use Mod::One;BEGIN {    use Mod::Two;    use Mod::Three; # will be loaded at the beginning of the BEGIN block before source is executed    print 1;}use Mod::Four;use Mod::Five;use Mod::Six; # Any uses are processed at compile time before anything else is executed
print 2; # Statements mixed in use lines are executed after all use's are doneprint 3;print 4; # from sub aprint 5; # from sub aprint 6;print 7; # from END

Only few lines are running if this file is used within a mod_perl or FastCGI installation. The second request will only execute these lines:

print 2;print 3;print 4; # from sub aprint 5; # from sub aprint 6;

The END-block (containing "print 7;") won't be executed until the Apache or FastCGI process exits.

Every Perl source file should be written in the same order Perl is executing the source:

#!/usr/bin/perl (if required) at the first line
use Modules; required to be loaded before the BEGIN block
BEGIN-block (if necessary, avoid it if every possible!)
use Modules; which require the BEGIN block to be loaded
Script source

Basic rules:

Don't ever write any source before or within use-lines and the BEGIN block (except source running within the BEGIN block).
Requiring a BEGIN block to load other modules is bad.

Visit Perldoc for the full Perl documentation of BEGIN, INIT, END.

If you're unsure about Perl's processing of your file, use Devel::Trace (perl -d:Trace yourscript.pl) or add some debug statements (but even they might not be executed in the order of their appearance).