The code simplifies to:

modifyHash(\%my_hash);

sub modifyHash
{
  my $hash_ref = shift;

  # Do something to the hash 
  ${$hash_ref}{'test'} = 1;
  #$hash_ref->{'test'} = 1;   #alternative
}
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basically, use {$hash_ref} wherever my_hash would have appeared:

%my_hash        -> %{$hash_ref}
keys(%my_hash)  -> keys(%{$hash_ref})
%my_hash = ();  -> %{$hash_ref} = ()
$my_hash{$key}  -> ${$hash_ref}{$key}
[download]