This works, but is SOOOO wrong....

It uses the debugger, and some little-documented side-effects of "caller"

#!/usr/bin/perl -w
package a; # the caller package

sub new {
 return bless {},shift
}

sub call {
 my ($self,$other)=@_;
 $other->do_something; # calling the other. Notice: no $self
}

sub back {
 my $self=shift;
 print "I've been called back!\n";
 print "And I have ",$self->{stuff},"\n";
}

package b; # the called package
use Data::Dumper;

sub new {
 return bless {},shift
}

sub do_something {
  my $p=DB::get_args();
  print Dumper($p); # this dumps the arguments of the caller (in this 
+case, a->call)
  $p->[0]->back()
}

package DB; # this MUST be called DB. It triggers magic in "caller"

sub get_args {
  my @a=caller(2); # 2 because 0 is this, 1 is the one that called it,
+ 2 is the one we need.
  # no, you can't remove the assignment: the optimizer will kill the l
+ine.
  return [@DB::args]; # @DB::args gets magically set to the param list
+.
}

package main;

$a=a->new;$b=b->new;
$a->{stuff}='something';
$a->call(b);
__END__
[download]

Will print:

$VAR1 = [
          bless( {
                   'stuff' => 'something'
                 }, 'a' ),
          'b'
        ];
I've been called back!
And I have something
[download]

UPDATE: clarified the meaning of the DB package