And, if you need to pass multiple parameters (eg. a hash and then an array) pass by reference; otherwise the hashes and arrays are flattened into a single list and the sub can't tell what is part of which.

Here is an example of using prototypes to force a pass-by-reference.

sub foo(\%\@)
{
   my ($hash_ref, $array_ref) = @_;
   #...
}

foo (%hash, @array);
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