You didn't specify anything about your data structure, so I made a guess at it.

use strict;
use warnings;


BEGIN {
   package Node;


   sub new {
      my ($class, $name) = @_;
      return bless({
         name => $name,
      }, $class);
   }


   sub name { $_[0]{name} }
}


BEGIN {
   package Label;


   sub new {
      my ($class, $name, $nodes, $branches) = @_;
      return bless({
         name     => $name,
         nodes    => $nodes,
         branches => $branches,
      }, $class);
   }


   sub name  { $_[0]{name}       }
   sub nodes { @{ $_[0]{nodes} } }


   sub flatten {
      my ($self) = @_;
      
      my @orphans;
      
      local *_flatten = sub {
         my ($self) = @_;
         our @nodes;    local *nodes    = $self->{nodes};
         our @branches; local *branches = $self->{branches};
         push @orphans, shift @nodes if @nodes == 1;
         @branches = map _flatten($_), @branches;
         return @nodes ? $self : @branches;
      };
      
      _flatten($self);
      push @{ $self->{nodes} }, @orphans;
   }


   sub visit {
      my ($self, $visitor) = @_;

      local *_visit = sub {
         my ($self, $depth) = @_;
         our @branches; local *branches = $self->{branches};
         $visitor->($self, $depth);
         for my $branch ( @branches ) {
            _visit($branch, $depth+1);
         }
      };

      _visit($self, 0);
   }
}


{
   sub node  { return Node->new(@_); }
   sub label { return Label->new(@_); }


   sub printer {
      my ($label, $depth) = @_;
      my $indent = '  ' x $depth;

      print("$indent+ ", $label->name(), "\n");

      for my $node ( $label->nodes() ) {
         print("$indent  = ", $node->name(), "\n");
      }
   }

   my $tree = (
      label('Label1',          [ node('Node1'), node('Node2') ], [
         label('Label2',       [ node('Node3')                ], [
            label('Label3',    [ node('Node4'), node('Node5') ], [
               label('Label4', [ node('Node6')                ], [
      ]) ]) ]) ])
   );

   $tree->visit(\&printer); print("\n");
   $tree->flatten();
   $tree->visit(\&printer); print("\n");
}
[download]

+ Label1
  = Node1
  = Node2
  + Label2
    = Node3
    + Label3
      = Node4
      = Node5
      + Label4
        = Node6

+ Label1
  = Node1
  = Node2
  = Node3
  = Node6
  + Label3
    = Node4
    = Node5
[download]

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