Restore the warning previously issued by (-l $fh)

This is a partial reversion of 433644eed8, which removed a
secondary, short-circuiting behavior when the warning was issued.

Now, the warning is issued, but the normal behavior (treat the
handle as a string) is maintained.  This work was done after
subsequent refactoring to doio.c, so it couldn't be just a
reversion with the "return" statement removed.