From perldoc perlrun:

       -w   prints warnings about dubious constructs, such as
            variable names that are mentioned only once and
            scalar variables that are used before being set,
            redefined subroutines, references to undefined file­
            handles or filehandles opened read-only that you are
            attempting to write on, values used as a number that
            doesn't look like numbers, using an array as though
            it were a scalar, if your subroutines recurse more
            than 100 deep, and innumerable other things.

            This switch really just enables the internal "^$W"
            variable.  You can disable or promote into fatal
            errors specific warnings using "__WARN__" hooks, as
            described in the perlvar manpage and the warn entry
            in the perlfunc manpage.  See also the perldiag man­
            page and the perltrap manpage.  A new, fine-grained
            warning facility is also available if you want to
            manipulate entire classes of warnings; see the warn­
            ings manpage or the perllexwarn manpage.
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Adding -w to your code should not affect whether or not your code runs; it should just give you a better idea why when it doesn't.

Another option is to use the use warnings pragma in your code.