my $tree = [ 1
           , [ 2
             , 3
             , [ 4
               , [ 5 ]
               , 6     ] ]
           , 7             ] ;

sub traverse {
    my ($nodefn, $tree, $depth) = @_;
    $depth ||= 0;
    $depth++;
    for (@$tree) {
        ref() ? traverse( $nodefn, $_, $depth )
              : $nodefn->( $_, $depth );
    }
}

##</code><code>##

sub count_nodes {
    my $count;
    traverse( sub { $count++ }, @_ );
    $count;
}

print count_nodes($tree), $/;
# 7

##</code><code>##

sub print_node {
    my ($leaf, $depth) = @_;
    print "  " x ($depth - 1), "* $leaf", $/;
}

##</code><code>##

traverse( \&print_node, $tree );
# * 1
#   * 2
#   * 3
#     * 4
#       * 5
#     * 6
# * 7

##</code><code>##

sub bfs_traverse {
    my ($nodefn, $tree, $depth) = @_;
    $depth ||= 0;
    $depth++;
    my @leaves   = grep !ref(), @$tree;
    my @branches = grep  ref(), @$tree;
    $nodefn->( $_, $depth ) for @leaves;
    bfs_traverse( $nodefn, [map @$_, @branches], $depth )
        if @branches;
}

##</code><code>##

bfs_traverse( \&print_node, $tree );
# * 1
# * 7
#   * 2
#   * 3
#     * 4
#     * 6
#       * 5