#!/usr/bin/perl -w

use strict;
use XML::Simple;
use Data::Dumper;

$/ = undef;
my $string = <DATA>;
my $ref = XMLin($string);
print Dumper $ref;

__DATA__

<config>
        <columnname>DATE extract</columnname> 
        <columnname>DATE_deleted</columnname>
        <columnname>DATE1</columnname> 
        <columnname>DATE2</columnname>
</config> 

##</code><code>##

$ perl xml.pl
$VAR1 = {
          'columnname' => [
                          'DATE extract',
                          'DATE_deleted',
                          'DATE1',
                          'DATE2'
                        ]
        };


##</code><code>##

my @columnames = @{$ref->{columnname}};

or loop over it without assignment.

printf "%s\n", $_ for (@{$ref->{columnname}});