Prototypes are determined at compile time and with that come several restrictions that make it a very bad fit for object-oriented constructs in perl. For one, it's terribly hard in specific cases and in general impossible for the perl compiler to determine an object's class at compile-time (i.e. by static analysis of the source code) so it won't be able to check which of any number of same-named functions will be used in a method invocation.

Also, perl's prototypes are really not about argument checking. They are about argument coercion - they don't really match what prototypes in C or Java for instance do.

See also Far More Than Everything You've Ever Wanted to Know about Prototypes in Perl