> > (Demonstrating the limitations of each is the most valuable outcome for me.)

> Really. That limitation is already well documented:

better phrased "The limitations on workarounds or alternatives in core".

The following code is tested and can handle sub-hashes of arbitrary length.

But using hashes and each is in the end a bad idea ... too inflexible, too many limitations, too risky in newer Perl-versions.

Using generated iterators OTOH could handle nested data-structures of any kind.

use Data::Dump qw'dd pp';
use strict;
use warnings;
use constant DEBUG =>0 ;
use feature 'say';

sub gen {
  my $ref=shift;
  keys %$ref;
  my @path=();
  return sub {
    while (1) {
      while (my ($k,$v) = each %$ref ) {
        if ( ref $v eq "HASH") {
          push @path,$ref;
          $ref =$v;
          keys %$ref;
          next;
        } 
        return $k,$v,scalar @path;
      }
      return unless  @path;
      $ref = pop @path; 
    }
  }
}



sub sameFringe {
  my ($h1,$h2)=@_;
  say "-------\n\n",pp $h1,$h2 if DEBUG;
  if ( $h1 != $h2 ){
    
    my $iter1=gen(shift);
    my $iter2=gen(shift);

    while ((my ($k1,$v1,$l1) = $iter1->()) + (my ($k2,$v2,$l2) = $iter
+2->()) ) {
      
      unless (defined $v1 and defined $v2 and $v1 eq $v2) {
        say "ne: ",pp {$l1,[$k1,$v1]},{$l2,[$k2,$v2]} if DEBUG >0;
        return "Different Fringe!";  
      }
      say "eq: ",pp [$l1,[$k1,$v1]],[$l2,[$k2,$v2]] if DEBUG >1;
    }
  }
  
  return 'Same Fringe!';
}


my $a = { l=> 1, r=>{ l=> 2, r=> { l=>3, r=> { l=>4, r=>5 } } } };
my $b = { l=> 1, r=>  { l=> { l=> 2, r=>  3 }, r=>  { l=> 4, r=>  5 } 
+} };
my $c = { l=> { l=> { l=> { l=> 1, r=>  2 }, r=>  3 }, r=>  4 }, r=>  
+5 };

say sameFringe($a,$a);
say sameFringe($a,$b);
say sameFringe($b,$c);
say sameFringe($a,$c);


my $x = { l=> 1, r=>  { l=> 2, r=>  { l=> 3, r=>  { l=> 4, r=>  { l=> 
+5, r=>  6 } } } } };
my $y = { l=> 0, r=>  { l=> { l=> 2, r=>  3 }, r=>  { l=> 4, r=>  5 } 
+} };
my $z = { l=> 1, r=>  { l=> 2, r=>  { l=> { l=> 4, r=>  3 }, r=>  5 } 
+} };

say sameFringe( $a, $x );
say sameFringe( $a, $y );
say sameFringe( $a, $z );
say sameFringe( $b, $x );
say sameFringe( $b, $y );
say sameFringe( $b, $z );
say sameFringe( $c, $x );
say sameFringe( $c, $y );
say sameFringe( $c, $z );
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I'll try to show a functional solution and a gather/take implementation in the coming days...