See the vars pragma docs for an explanation (Many Perl docs can be found on http://www.perldoc.com/).

In short, use vars qw($opt_h); declares the global variable $opt_h. In Perl one doesn't need to declare variables prior to using them, but for anything non-trivial it's strongly advised to do so nevertheless. This can be enforced by using the strict pragma (use strict;), from this line on, all variables must be declared with my or our (see the docs for the strict pragma).

Hope this helps, -gjb-