Is there someplace I could find out what ENV keys I should be expecting in general...

Well, do you have access to each of the systems where this script is expected to run? If so, just run this command line on each system:

perl -e 'print map { "$_ = $ENV{$_}\n" } sort keys %ENV'
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Another reply below has already mentioned %ENV keys that are specific to CGI, and any one of these should be common across systems...