# BAD
sub outer {
   my $var;
   ($t, @y) = rkXXX(\@y, $dydt, $t, $dt);
   my $dydt = sub { ... };
}

# Good
sub outer {
   my $var;
   my $dydt = sub { ... };
   ($t, @y) = rkXXX(\@y, $dydt, $t, $dt);
}