#!/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 ) };
}