Nothing stopping you from doing that:

my $sub = sub {
 . . . 
};
my $obj1 = Foo->new( $sub );
my $obj2 = Foo->new( $sub );
[download]

I meant two different modules that want to use the same sub ref.

Hmm, in that case you might be constrained by your design, but it is still possible like this:

package Foo;
sub filter 
{
    . . . 
}

package Bar;

my $obj = Baz->new( \&Foo::filter );
[download]

Admittedly, this could get hackish fast.

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

: () { :|:& };:

Note: All code is untested, unless otherwise stated