The entire playlist of Russ doing AoC this year has been great. Using a combination of ivy, perl, awk, sam, and acme. Normally I focus on writing small scripts or programs to complete each day but using a REPL or similar interactive environment to reach the answer is a neat solution I had not thought of.

Telling Perl to REPL is absolutely insane:

perl -e 'while(<>) { print eval, "\n\n" }'

perl -dE 0

Puts you at the step debugger command line, effectively a REPL.

Or use reply: https://metacpan.org/dist/Reply/view/bin/reply

Or use re.pl, included with Devel::REPL: https://metacpan.org/pod/Devel::REPL