my $key = "0123456789ABCDEF0123456789ABCDEF";

local $_ = $key;
s/^0+//;
$_ = "0$_" if length() % 2 == 1;
$_ = "00$_" if /^[89ABCDEFabcdef]/;
my @b = unpack('c*', pack('H*', $_));