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use Data::Diver qw( DiveVal );

...
  -or-
my %branch;
DiveVal(\%branch, map \$_, @keys) = 'leaf';

sub DiveVal :lvalue {
   my $p = \shift;
...
  -or-
my %branch;
DiveVal(\%branch, @keys) = 'leaf';

Pre-loop:          $p references $branch
After loop pass 0: $p references $branch->{level1}
After loop pass 1: $p references $branch->{level1}{level2}
After loop pass 2: $p references $branch->{level1}{level2}{level3}
Returned:          $branch->{level1}{level2}{level3}