$ cat x.pl 
use warnings;
use strict;
sub x {
	my $y = 123;
	sub {
		$y += shift;
		print "$y\n";
		print shift->(), "\n";
	}
}
my $z = x;
my $foo = "abc";
$DB::single=1;
$z->(111, sub {
	return $foo."def";
});

$ perl -d x.pl 
...
main::(x.pl:11):	my $z = x;
  DB<1> c
main::(x.pl:16):	});
  DB<1> s
main::CODE(0x543ed97f20a0)(x.pl:6):
6:			$y += shift;
  DB<1> y 0
$y = 123
  DB<2> s
main::CODE(0x543ed97f20a0)(x.pl:7):
7:			print "$y\n";
  DB<2> y 0
$y = 234
  DB<3> s
234
main::CODE(0x543ed97f20a0)(x.pl:8):
8:			print shift->(), "\n";
  DB<3> s
main::CODE(0x55d4d6a31da8)(x.pl:15):
15:		return $foo."def";
  DB<3> y 0
$foo = 'abc'
$z = CODE(0x543ed97f20a0)
   -> &main::__ANON__[x.pl:9] in x.pl:5-9