sub func {
   my ( $rs_str, $hr_desktop_info, $ha_files ) = @_; 

  $$str = update_string_ref();
  for ( keys %$desktop_info ) {
     ...
     push @$files, $file;
  }
  ....

}
[download]

I rather feel that $$str, %$desktop_info and @$files make it pretty clear, not only that your dealing with references, but also what type of references they are.

If you're having problems reading that, I suggest you do what ++stevieb has already alluded to and put the prefixes in a comment. Something like:

    my ($str, $desktop_info, $files) = @_; # rs, rh, ra
[download]

Update (minor typo fix): s{deck}{desk} in ..., %$decktop_info and ....