warn LIST
            Prints the value of LIST to STDERR. If the last element of LIST
            does not end in a newline, it appends the same file/line number
            text as "die" does.

            If the output is empty and $@ already contains a value
            (typically from a previous eval) that value is used after
            appending "\t...caught" to $@. This is useful for staying
            almost, but not entirely similar to "die".

            If $@ is empty then the string "Warning: Something's wrong" is
            used.

            No message is printed if there is a $SIG{__WARN__} handler
            installed. It is the handler's responsibility to deal with the
            message as it sees fit (like, for instance, converting it into a
            "die"). Most handlers must therefore arrange to actually display
            the warnings that they are not prepared to deal with, by calling
            "warn" again in the handler. Note that this is quite safe and
            will not produce an endless loop, since "__WARN__" hooks are not
            called from inside one.

            You will find this behavior is slightly different from that of
            $SIG{__DIE__} handlers (which don't suppress the error text, but
            can instead call "die" again to change it).

            Using a "__WARN__" handler provides a powerful way to silence
            all warnings (even the so-called mandatory ones). An example:

                # wipe out *all* compile-time warnings
                BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
                my $foo = 10;
                my $foo = 20;          # no warning about duplicate my $foo,
                                       # but hey, you asked for it!
                # no compile-time or run-time warnings before here
                $DOWARN = 1;

                # run-time warnings enabled after here
                warn "\$foo is alive and $foo!";     # does show up

            See perlvar for details on setting %SIG entries and for more
            examples. See the Carp module for other kinds of warnings using
            its carp() and cluck() functions.