-X FILEHANDLE
-X EXPR
-X A file test, where X is one of the letters listed below. This unary
operator takes one argument, either a filename or a filehandle, 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, and
the argument may be parenthesized like any other unary operator. The
operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
....
-M Script start time minus file modification time, in days.
....
