Surely a lot of the blame for that lies with h2xs, which casually inserts the current version of perl at the top of any created .pm files. Although admittedly the final responsibility for changing this lies with the author, I think too many people (myself included? never!) just think "isn't everyone running perl 5.020?", and leave it the way it is - any dependencies obviously work with that version, and to figure out the absolute minimum is Just Too Much Work.