my $key = "0123456789ABCDEF0123456789ABCDEF";

local $_ = $key;
s/^0+//;
$_ = "0$_" if length % 2 == 1;
$_ = "00$_" if /^[89ABCDEFabcdef]/;
my @b = map { chr hex $_ } /(..)/g;