In the second snippet, $cnt++ is only executed when $cnt is not 4. Learn how to indent properly to avoid these errors.

The arguments for rand are off by one. Random returns a number from 0 (inclusively) to arg (exclusively). You need 48 and 256. Better yet, use hex (0x30 and 0x100) since it's more meaningful here.

I have no idea what if ($rand < 5) { $rand += 2; ... is all about. If you want to avoid '00' and '01', use

rand(0x100 - 2) + 2</p>

<p>Why not just use</p>

<c>
sub mac {
   return join ':',
      '00:96:14',
      sprintf('%02X', rand(0x30)),
      sprintf('%02X', rand(0x100)),
      sprintf('%02X', rand(0x100));
}
[download]

or

sub mac {
   return sprintf('00:96:14:%02X:%02X:%02X',
      rand(0x30),
      rand(0x100),
      rand(0x100),
   );
}
[download]

A loop isn't appropriate here due to the lack of commonality between the octects