A node I wrote much earlier show how to convert a MAC address from a hex string into a floating point number, since they have more bits of precision. Just be sure not to use integer operations on the number. If you can figure out how to do a large random, here's a harness:

# IMPORTANT:
# $first_mac, $last_mac, $range and $random
# are floating point number potentially too
# big to fit into an integer. Do not perform
# integer operations on them.

my $first_mac = mac_hex2num('00:96:14:00:00:00');
my $last_mac  = mac_hex2num('00:96:14:2F:FF:FF');

my $range = $last_mac - $first_mac + 1;

my $random = ???;

my $mac = man_num2hex($first_mac + $random);
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