Close, but not quite. You're trying to store a 48bit number into a 32bit variable. (Well, it's 32 bit on my Windows machine.) You have an overflow problem.

my $dec = hex('FF0000000000');
printf("%012x", $dec);
__END__
Integer overflow in hexadecimal number at !.pl line 1
0000ffffffff
[download]

You could use Math::BigInt, or you could use floating point numbers (doubles) as I've previously detailed.