$0 and __FILE__ are subtly different. $0 will only ever contain the name of the program that is running. If the program calls code in other files (modules, for example) then __FILE__ will have different values.

These values are documented in the section on "Special Literals" in perldata.

The distinction between $0 and __FILE__ is handy. You can use it to implement unit tests for classes. I do this a lot in Ruby. For example, in classFoo.pm:

#/usr/bin/perl
package Foo;

sub new {
  # ...
}

# other stuff

if (__FILE__ eq $0) {
  my $foo = new Foo;
  # test foo's methods, etc
}

1;
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Now you'll only get the unit tests if you run classFoo.pm directly, because otherwise, __FILE__ ne $0.

Of course, I must recommend using Test::More and friends instead. Just showing a possible application of the subtle difference between __FILE__ and $0.

caller is more reliable for this.

#!/usr/bin/perl

package Foo;

sub new {
  # ...
}

# other stuff

if (!caller()) {
  my $foo = new Foo;
  # test foo's methods, etc
}

1;
[download]