I'm not sure what proof you have that your strings are reverting. Be aware that both @contents and $s are the results of calling a method on an object. If you are expecting a substitution on a string returned by contents_list to be refs, then then you really want content_refs_list instead. The HTML::Entity method contents_list only returns a list, not a list of refs. It seems you're changing something other than what you want.

As for being able to change your arguments in Perl, take a look at this simple test case:

my @foo = qw( aaaaaaaa bbbbbbbb cccccccc dddddddd eeeeeeee ffffffff);
sub foo {
  for ( @_ ) {
      $_ =~ s/(.)\1/$1/ || return;
      print $_ . "\n";
      foo( $_ );
  }
}

foo @foo;
print @foo;
[download]

The above produces this: