The documentation for the file test operators is in perlfunc under -X, which can be a little tricky to track down unless you were specifically looking for the -X operator.

Alternatively, from the commandline, it's a little easier (although these still all take you the the same doco):

$ perldoc -f -M
$ perldoc -f -A
$ perldoc -f -C
$ perldoc -f -X
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