use feature say;

$_ = "012\xf0ABC\x0f\x00\x01\xff";
say unpack "H*";    #  303132f04142430f0001ff
for (my $i = length; $i--; vec($_, $i, 8) ^= ~0) {}
say unpack "H*";    #  cfcecd0fbebdbcf0fffe00