Without sitting down to write the code, I would approach this from a piece environment. I would just split(/./,$IP,4) and then calculate the differences between each octet of the two IPS and then multiply to find the number of IPs generated. Something like:

$OctDifference1*($OctDifference2*($OctDifference3*$OctDifference4));
[download]

Also I would increment the octets spearately and then cat them together.
HTH
CiceroLove

UPDATE:Or you could be much more elegant and faster like Randall was whilst I was typing my crummy response. :O)