The tools have been provided already, but I wanted to add a comment: You can't do it perfectly unless you disallow certain special cases related to overloading. Perl is designed around of the concept of implicit coercion, so trying to use type-based polymorphism in Perl is inherently flawed.

By the way, your use of a prototype is not only unnecessary, it's wrong. You tell Perl that the sub takes no arguments ("sub xyz()"), but the sub clearly does take arguments ("while(defined($arg = shift))") and you have to tell Perl to ignore the fact that you told it the sub takes no arguments ("&xyz"). You should have

sub xyz {
   ...
}

xyz($s);
xyz(\%h);
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