Use object overloading.

Ties and Variable::Magic (which was mentioned elsewhere in the thread) attach to the variable, not to the value, like this:

my $x = "Hello world";

attach_some_special_magic( $x );

my $y = $x;

# $y has no special magic

As you're passing the string around, you don't want that. You want an overloaded object.

The danger with overloaded objects is that the third-party module you mention might check !ref( $str ) to ensure it's been passed a string. If it's doing that, you still have one workaround. Bless the object into a package called "0". Then ref( $str ) will return "0" which is false.