Why concatenate a space to the key? my $key = join'',@keys;

That isn't a space, it's the null string. Ie. Two single quotes, with nothing in between.

and Why $answer = 'The' . $answer; NP1 have 'the'core even it is not in uppercase the first character.

That's because I didn't recognise that each /tag applied to everything between it and the preceeding tag. My mistake. I'll amend the code above in a few minutes.