1. is pretty easy,

$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) );
[download]

I simplified the end of array indexing a bit.

You can view the results with

print unpack 'H*', join '', @lock_arr;
print unpack 'H*', join '', @key_arr;
[download]

You appear to have split the lock phrase into an array for convenience. Another approach would be with substr. I'd combine both operations in a single sub which takes the lock string or array as an argument and returns the key:

sub keycalc {
    my @key = @_;
    for (0..$#key) {
        $key[$_] = pack 'h2', unpack 'H2',
            ($_ ? $key[$_] ^ $key[$_ - 1]
#                : $key[-1] ^ $key[-2] ^ 5); # same error
                : $key[-1] ^ $key[-2] ^ chr(5));
    }
    @key;
}
print unpack( 'H*', join '', @lock_arr), $/;
print unpack( 'H*', join '', keycalc( @lock_arr)), $/;
[download]

Update: Oops! you need the numeric 5 as a character, Fixed with chr, as indicated.

After Compline,
Zaxo