does not leave around refs to empty hashes (it leaves undef's instead).

What does that mean? In fact, vivification is required here.

use strict;
use warnings;

use Data::Dumper qw( Dumper );

sub samtregar {
   my $p = shift;
   my $val = pop;
   for my $item (@_) {
      $p->{$item} ||= {};
      $p = $p->{$item};
   }
   $p->{_val} = $val;
}

sub repellent {
   my $p = \shift;
   my $val = pop;
   $p = \$$p->{$_} for @_;
   $$p->{_val} = $val;
}

{
   samtregar(my $samtregar={}, qw( a b c d ), 'foo');
   repellent(my $repellent,    qw( a b c d ), 'foo');

   {
      local $Data::Dumper::Useqq  = 1;
      local $Data::Dumper::Terse  = 1;
      local $Data::Dumper::Indent = 0;
      print("samtregar: ", Dumper($samtregar), "\n");
      print("repellent: ", Dumper($repellent), "\n");
   }
}
[download]

samtregar: {"a" => {"b" => {"c" => {"d" => {"_val" => "foo"}}}}}
repellent: {"a" => {"b" => {"c" => {"d" => {"_val" => "foo"}}}}}
[download]

And for a getter, what you said is backwards. Yours is the one that vivifies.

use strict;
use warnings;

use Data::Dumper qw( Dumper );

sub samtregar_fetcher {
   my $p = shift;
   for my $item (@_) {
      last if !$p->{$item};
      $p = $p->{$item};
   }
   return $p->{_val}
}

sub repellent_fetcher {
   my $p = \shift;
   my $val = pop;
   $p = \$$p->{$_} for @_;
   return $$p->{_val};
}

{
   samtregar_fetcher(my $samtregar={}, qw( a b c d ));
   repellent_fetcher(my $repellent,    qw( a b c d ));

   {
      local $Data::Dumper::Useqq  = 1;
      local $Data::Dumper::Terse  = 1;
      local $Data::Dumper::Indent = 0;
      print("samtregar: ", Dumper($samtregar), "\n");
      print("repellent: ", Dumper($repellent), "\n");
   }
}
[download]

samtregar: {}
repellent: {"a" => {"b" => {"c" => {}}}}
[download]

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