The loop would be better written:

for my $sub (@{$Document->find('PPI::Statement::Sub') || []}) {
    next if $sub->forward;

    for my $child ($sub->children) {
        if (ref $child eq "PPI::Structure::Block") {
            $child->start->add_content("my mars code");
        }
    }
}
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The next if ... saves a level of indentation and is easier to understand (at least for me) than an unless.

Using a Perl style for loop in place of the C style for loop is easier to read, easier to understand, more compact and much less likely to get wrong.