If you want to override a predefined function, such as open,
then you'll have to import the new definition from a different module. See Overriding Builtin Functions. There's also an example in Class/Template.
If you want to overload a Perl operator, such as + or **, then you'll want to use the use overload pragma, documented in the overload manpage.
If you're talking about obscuring method calls in parent classes, see Overridden Methods.