I would take a serious look at the timeline for the rewrite before hiring someone to stabilize what will, eventually, be completely thrown away. If you expect the rewrite to take, say, 6 man-months, then hire 3 Perl consultants and take the 2 month hit.

If, on the other hand, you're planning for a man-year or more, I'd take a deeper look at your rewrite plans. Maybe they're too ambitious?

As for how to hire a consultant ... as a Perl consultant myself, just make sure that the person (or people) have the Perl skills you require and that s/he has the project debugging skills you require. That's all that was done when hiring me (and the other 6 Perl consultants in the shop I'm at). I've done two technical interviews already and that's what I was directed to inquire about.

On a more introspective note, I'd personally go after your previous manager and shoot him. Encouraging static temp tables and polluting main:: should be reasons for justifiable manslaughter. :)