I am pretty sure that is what I have:

You suggested, "{ if ( exists $new_list{$file} ) {"

The code is broken into two lines to improve readability, but all of the code you suggested is present. Unless I am missing something!

sub DirCompare{
  
  for my $file ( keys %old_list ) {
    if ( exists %new_list{$file} ) {
       next;
     }
    else {
      push @diffs, "Old file not in new: $file";
    }
  }
  
  for my $file ( keys %new_list) {
    if ( exists %old_list{ $file } ) {
      next;
    }
    else {
      push @diffs, "Old file not in new: $file";
    }
  }
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