If you want to provide a compile-time check that parameters passed to a function are of the correct type, use a prototype:

sub functionname($@$) { ... }
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The problem with this example is that perl flattens the @ and second $ parameter into one list, so your function wouldn't know where the second parameter (the @) ends.

There are some techniques in the cookbook and other literature that you might want to investigate, such as mimicking behaviour of built-ins with function prototypes. Some of these techniques can be used to alleviate the problem I've identified above.

You may also think about having your function accept only a hash (actually, a list of arguments your function will treat as a hash), and requiring callers to use named parameters:

sub functionname
{
  my %hash = @_;
  # Validate keys of hash here
} 


functionname(param1 => 1, param2 => 2, ...);
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Did I mention There's More Than One Way To Do It? ;)