TL;DR

Challenge 2 in Cryptopals.

Challenge 2 is almost trivial in Perl, thanks to how operator ^ works. Or I should probably say operator ^., which is enabled by default starting from perl v5.28 on (or use feature "bitwise"; starting from perl v5.22).

So here we go:

sub fixed_xor ($string1, $string2) { $string1 ^. $string2 }

As indicated in the challenge, of course we have to do a bit of back and forth with decoding the input (provided in hex form) and encoding it back before printing it:

my $in1 = '1c0111001f010100061a024b53535009181c';
my $in2 = '686974207468652062756c6c277320657965';
my $out = '746865206b696420646f6e277420706c6179';

say encode_base16(fixed_xor(decode_base16($in1), decode_base16($in2)));
say $out;

Function encode_base16 and decode_base16 are the same from the previous post Cryptopals 1 - Convert hex to base64.

Stay safe and secure!