perlgolf

Thanks to Michael Schrijver for the writeup!

PREHISTORIC EXTRACTION AND REPORTING LANGUAGE

The flag is constructed using the perl one-liner below:

echo $FLAG | perl -ple 'y/a-z0-9/0-9a-z/;s,.,$&^"\x31",eg;s/./sprintf("%02d",ord($&))/eg;$/=reverse;tr/0-9/5812907643/'
5942585669617662686874626877526268765151507474557752525959517477585160606567

Using flag -e means the script will be specified as an argument, -p wraps the script in loop reading stdin and printing to stdout. I think -l enables line buffered mode, didn't check.

Breaking down the script into pieces:

y/a-z0-9/0-9a-z/

    y is a synonym for tr. It does character translation

s,.,$&^"\x31",eg

    The 'e' flag to regex means interpret substitution as expression, the 'g' applies it all matches and not just the first one. The expression xors the match with 0x31.
    Effectively this just xors the string with 0x31. In Perl you're not limited to / to separate the regex pieces, here a comma is used to confuse the reader.

s/./sprintf("%02d",ord($&))/eg

    Again an expression substitution, this time printing each character as a decimal number zero-padded to fill two characters.

$/=reverse

    Doesn't do anything, I didn't understand the code but found out through trying.

tr/0-9/5812907643/

    Another character translation.

So we reverse the order of the pieces and invert each piece.
For the character translations you just swap around first and second argument. Xor works both ways.
The decimal character encoding is undone by matching two characters and replacing them with chr(match).

Ending up with:

    echo 5942585669617662686874626877526268765151507474557752525959517477585160606567 | perl -ple 'tr/5812907643/0-9/;s/../chr($&)/eg;s,.,$&^"\x31",eg;y/0-9a-z/a-z0-9/'
    flag{817554752c751dde44b2ccffd42ad996}