Hidden four hasn’t got a description.
As hidden four came out the same day as HV19.14 Achtung das Flag I thought it must be about that challenge.
As the flag from HV19.14 looks pretty messed up, and as the code states at some point that Only perl can parse Perl!, I thought, maybe just run the flag from HV19.14 with perl.
s@@jSfx4gPcvtiwxPCagrtQ@,y^p-za-oPQ^a-z\x20\n^&&s[(.)(..)][\2\1]g;s%4(...)%"p$1t"%ee
Running this prints Squ4ring the Circle. So the flag for Hidden Four is HV19{Squ4ring the Circle}.