Excuse me, but you're not being fair here. I really don't know why you need to re-assign to the $o1 and $o3 objects:

# ... when you call Forsaken_clean
$o1 = $o1->Forsaken_clean;


# ... and when you call rev_1318_clean
$o3 = $o3->rev_1318_clean;
[download]

The assignment risks to spoil the left hand side without added value: the call itself already cleans up the objects. And it really does spoil $o3, as we can see comparing the cleaning functions:

   sub Forsaken_clean{
       my $self = shift;
       foreach (keys %$self) {
           delete $self->{$_};
       }
       $self;
   }
   sub rev_1318_clean{
       my $self = shift;
       %$self = ();
   }
[download]

The former has a $self as last statement, while the latter hasn't and it's returning nothing. Fixing is trivial, just add a line in rev_1318_clean:

   use strict;
   use Data::Dumper; $Data::Dumper::Indent = 1;
   
   my $o1 = gugus->new;
   my $o2 = $o1;
   print 'This is before Forsaken_cleaning: ', Dumper $o1, $o2;
   
   $o1 = $o1->Forsaken_clean;
   print "\n", 'This is after Forsaken_cleaning: ', Dumper $o1, $o2;
   
   my $o3 = gugus->new;
   my $o4 = $o3;
   print "\n", 'This is before rev_1318_cleaning: ', Dumper $o3, $o4;
   
   $o3 = $o3->rev_1318_clean;
   print "\n", 'This is after rev_1318_cleaning: ', Dumper $o3, $o4;
   
   package gugus;
   sub new {
       my $class = shift;
   
       my $self  = { 'k1' => 4711 };
   
       bless $self, $class;
   
       return $self;
   }
   sub Forsaken_clean{
       my $self = shift;
       foreach (keys %$self) {
           delete $self->{$_};
       }
       $self;
   }
   sub rev_1318_clean{
       my $self = shift;
       %$self = ();
       $self;   # This is the added line
   }
   
   __RESULT__

This is before Forsaken_cleaning: $VAR1 = bless( {
  'k1' => 4711
}, 'gugus' );
$VAR2 = $VAR1;
 
This is after Forsaken_cleaning: $VAR1 = bless( {}, 'gugus' );
$VAR2 = $VAR1;
 
This is before rev_1318_cleaning: $VAR1 = bless( {
  'k1' => 4711
}, 'gugus' );
$VAR2 = $VAR1;
 
This is after rev_1318_cleaning: $VAR1 = bless( {}, 'gugus' );
$VAR2 = $VAR1;
[download]

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