use warnings;

sub outer {
   my ($var) = @_;

   sub inner {
      print("$var\n");
   }

   inner();
}

outer("abc");
outer("def");

##</code><code>##

Variable "$var" will not stay shared at line 7.
abc
abc

##</code><code>##

use Math::RungeKutta;

sub outer {
   my $var = ...;

   local *dydt = sub {
      ... $var ...
   };

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

##</code><code>##

use Math::RungeKutta;

sub outer {
   my $var = ...;

   my $dydt = sub {
      ... $var ...
   };

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