for (0..$#subnets) {
    # take each subnet and generate the hex out of the subnet divided 
+by 16
    # then the remainder
    my $pair = hexize((int($subnets[$_]/16)),($subnets[$_]%16));
    print $pair;
}
print "\n";
[download]

sprintf makes it easier:

print hexize(@subnets), "\n";

sub hexize {
    return sprintf("%02x%02x%02x%02x", @_);
}
[download]

Obviously, Net::IP makes things much easier, as anonymonk pointed out.