Advanced Perl Programming

Package Initialization and Destruction

There are times when you want to do some initialization before any other code is executed. Perl goes further: it gives you a chance to execute code while it is still in the compilation stage.

Normally, while parsing a file, Perl compiles the entire code, and when this process is successfully completed, it starts executing from the first global statement onward. However, if it encounters a subroutine or a block called BEGIN while parsing, it not only compiles it, but also executes it right away, before resuming the compilation of the rest of the file. A small experiment underscores this point:

sub BEGIN {   # can also just say BEGIN { }; the word "sub" is optional
    print "Washington was here \n";
}
foo***  ;     # Intentional error

This prints the following:

Washington was here 
syntax error at x.pl line 4, near "**  ;"
Execution of x.pl aborted due to compilation errors.

Whereas a program with a syntax error normally does not get executed at all, a BEGIN subroutine occurring before the error will be executed.

Because a BEGIN block gets executed even before the compilation phase is over, it can influence the rest of the compilation. If you want to hardcode an include path in your program, here is how to do it:

BEGIN {
    unshift (@INC, "../include");
}
use Foo;  # Looks for Foo.pm in "../include" first

An easier approach is to use the lib module that is packaged with the Perl distribution:

use lib qw(../include); # prepends the directory to @INC

Just as you ...