Thanks for the times. But just to balance it out, yours could easily be considered the most straightforward:

(cleaned up a little):     $str =~ s/..(?!$)/$&:/g;

says "insert a colon between each pair of characters" at least as clearly as any of the others. Slight update: That should be s/..(?=..)/$&:/g, to make the point about readability.

  p