From which I gather the filetests don't take a default "$_".

Well if you read "perldoc -f -X" you'll see it says:

    -X FILEHANDLE
    -X EXPR
    -X DIRHANDLE
    -X      A file test, where X is one of the letters listed below.  This unary
            operator takes one argument, either a filename, a filehandle, or a
            dirhandle, and tests the associated file to see if something is true
            about it.  If the argument is omitted, tests $_, except for "-t", which
            tests STDIN.  Unless otherwise documented, it returns 1 for true
            and '' for false, or the undefined value if the file doesn't exist.  Despite
            the funny names, precedence is the same as any other named unary operator.