perldoc perlrun

##</code><code>##

       -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
            filehandles or filehandles opened read-only that you are attempt-
            ing 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 sub-
            routines recurse more than 100 deep, and innumerable other things.