use strict;
use warnings;
use PPI;

my $file = '/path/to/perl/script.pl';
my $Document = PPI::Document->new($file) or die "oops";

for my $sub ( @{ $Document->find('PPI::Statement::Sub') || [] } ) {
    unless ( $sub->forward ) {
        my @elements = $sub->children;

        for ( my $i = 0 ; $i < @elements ; $i++ ) {
            if ( ref $elements[$i] eq "PPI::Structure::Block" ) {
                $elements[$i]->start->add_content("my mars code");
            }
        }
    }
}

$Document->save($file.".new");