package Slave;
use Master;

##</code><code>##

package Master;
use strict; use warnings;
sub import {
    my $target = caller;
    no strict 'refs';
    *{"$target\::nx"} = sub { do_something() }
}

##</code><code>##

package Slave;
use strict; use warnings;
use Master;
sub do_something { my $pkg=caller(); print "$pkg\n"; }

##</code><code>##

eval "package $caller; sub nx { do_something() }";