Having been in a similar situation to yourself (5.00x vs 5.6+) I'd say target the former. While you get a few syntactic niceties (lexical filehandles, use warnings etc) there's not a great of difference between the two in terms of language. Your programs won't benefit hugely from 5.6+ so you might as develop for the lowest common dominator (which isn't even that low in this case). And besides our() is horrid.
HTH

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broquaint