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$key_arr[0] = pack 'h2', unpack 'H2',
     ( $lock_arr[0] ^ $lock_arr[-1]
#       ^ $lock_arr[-2] ^ 5 ); # not quite right
       ^ $lock_arr[-2] ^ chr(5) );

print unpack 'H*', join '', @lock_arr;
print unpack 'H*', join '', @key_arr;

sub keycalc {
    my @key = @_;
    for (0..$#key) {
...
}
print unpack( 'H*', join '', @lock_arr), $/;
print unpack( 'H*', join '', keycalc( @lock_arr)), $/;