It is basically the same thing as the last "thing" tested with the previous test operator but without having to re-test it (which is apparently expensive). I remember reading that in a book.

Update: I think it is called 'chaining' tests.

Update2:

To save the overhead of a low-level stat() system call, Perl caches the previously-examined inode in a special variable called simply '_'; so we can do tests like this:

if ( -e $file && -r _ && -x _) {
   # $file exists and is both readable and executable
   ...
}
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