Update: HA! boftx posted almost exactly this (and I endorse the __END__ bit as well) while I didn't refresh the OP to notice... Great minds, I say!

In addition to what roboticus said, here's a skeleton of what you are suggesting-

#!/usr/bin/env perl
use strict;
use warnings;

main_routine();

exit;

sub main_routine {
    # lots of stuff.
}
[download]

Keeping your subs separate with an exit keeps code accidents away pretty well.