Here's a relatively easy way I pounded out in about 2 minutes...

$match = 0;
$ip = $ARGV[0] ? $ARGV[0] : "129.10.10.120";
($a,$b,$c,$d) = split '\.', $ip;

$match = 1 if ($a > 127 && $a < 256);
print "$match\n";
[download]

Nothing glamorous, that's for sure, but it's an easy comparison.