I made a new cygwin installation just for using Perl, and did not install Perl, nor anything that included Perl (which was restrictive and extremely tedious, as the Cygwin installer doesn't list package dependencies or say which package required an auto-added inclusion). Then I installed Strawberry Perl on the system, outside of Cygwin. I didn't have to set any environment variables. Cygwin automatically detects and uses Strawberry Perl if no other perl is present. I will probably put Strawberry Perl at the front of my PATH so that I will no longer have to check the auto-include list everytime I update cygwin to make sure nothing depends on Perl (and thus will install a Cygwin perl.)

I still would like to know if anyone out there has gotten the latest Cygwin release of Perl to work.