It means:
Print "Dohhh!" if the file size is non-zero for the file that most recently had a file test or 'stat' done on it. That is, if the previously tested file has anything at all in it.

See perlfunc:_X for the '-s' file test and, at the very end, the use of the 'solitary underscore' for referring to the previously tested file.

In reply to Re: Conditional Wierdness by dvergin
in thread Conditional Wierdness by jlongino

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