Quoth  perldoc -f stat

If "stat" is passed the special filehandle consisting of an
underline, no stat is done, but the current contents of the stat
structure from the last "stat", "lstat", or filetest are
returned. Example:

    if (-x $file && (($d) = stat(_)) && $d < 0) {
        print "$file is executable NFS file\n";
    }
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