use Math::RungeKutta;

sub outer {
   my $var = sub_from_other_package();
   no warnings qw(closure);

   sub differ {
      my $y = shift;
      $var->[...]{...}{...}; etc.
      ...
      return $res;
   }

   sub dydt {
      my ($t,@y) = @_;
      return differ($y);
   };

   ($t, @y) = rkXXX(\@y, \&dydt, $t, $dt);

}