This is from the Perl Cookbook Recipe 11.17 (Program: Binary Trees).

#!/usr/bin/perl

use strict;
use Data::Dumper;

my $root;

insert( $root, 5 );
insert( $root, 3 );

print Dumper \$root;

sub insert {
    my ($tree, $value) = @_;

    unless ($tree) {
        $tree = {};
        $tree->{VALUE} = $value;
        $tree->{LEFT} = undef;
        $tree->{RIGHT} = undef;
        $_[0] = $tree;             # $_[0] is reference param!
        return;
    }

    if ( $tree->{VALUE} > $value ) { insert( $tree->{LEFT}, $value ) }
    elsif ( $tree->{VALUE} < $value ) { insert( $tree->{RIGHT}, $value
+ ) };
}
[download]

I am trying to understand the relevance of this line:

$_[0] = $tree;
[download]

He mentioned that this is a reference param. And also he mentioned that "The assignment of the new node back to $_[0] alters the value in its caller."

But I still don't get it. If I removed this line, I don't get any output.

I ran the debugger but to no avail, I still could not figure out what is the relevance of that line. Why did he write it? And is there another way to write it maybe a longer version?

Can someone help understand this? Thanks.

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