Fix perl interp path for perlversions other than system-perl

Undoing part of 4d5f81aafd68d6e9542168e2b622eaf026669f8c which assumed
that the perl interpretter for X.Y.Z is *always* perlX.Y rather than
perlX.Y.Z. That's only true for system-perl, which is only one
specific (but different per kernel) variant on each OS X. Fink
supplies other interp versions, and uses the fully-versioned
interpretter name.