The only thing that can be passed to a function is a list of scalars. Solutions:

• You could pass a reference to the hash and use that:

  sub dump_hash {
     my ($hash) = @_;
     foreach my $key (keys %$hash) {
        print $key, '=>', $hash->{$key}, "\n";
     }
  }
  
  dump_hash(\%hash);
  [download]

• You could convert the hash to a list, then recreate the hash:

  sub dump_hash {
     my %hash = @_;
     foreach my $key (keys %hash) {
        print $key, '=>', $hash{$key}, "\n";
     }
  }
  
  dump_hash(%hash);
  [download]