Only scalars can be passed as arguments, so either need to pass a reference (like in the first and second snippet), or flatten the hash into a list of scalars (like in the third snippet).

One usually passes a reference to the hash which saves Perl from copying the entire hash:

sub function {
   my ($hash_ref) = @_;

   foreach (keys %$hash_ref) {
      ...
   }
}

function(\%hash);
[download]

If you don't with to work with references, you could use an alias:

sub function {
   our %hash; local *hash = \$_[0];

   foreach (keys %hash) {
      ...
   }
}

function(\%hash);
[download]

Or you could create a copy of the hash if it's the last argument:

sub function {
   my %hash = @_;

   foreach (keys %hash) {
      ...
   }
}

function(%hash);
[download]