/molecule_idref="([^"]+)/

##</code><code>##

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

my $xml;
{
    local $/='';
    $xml = <DATA>;
}

my $xs = XML::Simple->new();
my $ref = $xs->XMLin($xml);

print Dumper($ref);

__DATA__
<xml><ComplexComponent1 molecule_idref="1"/>
<ComplexComponent2 molecule_idref="2"/><ComplexComponent3 molecule_idref="3"/>
<ComplexComponent4
molecule_idref="4"/><ComplexComponent5
molecule_idref="5"/>
</xml>

##</code><code>##

$VAR1 = {
          'ComplexComponent3' => {'molecule_idref' => '3'},
          'ComplexComponent5' => {'molecule_idref' => '5'},
          'ComplexComponent1' => {'molecule_idref' => '1'},
          'ComplexComponent4' => {'molecule_idref' => '4'},
          'ComplexComponent2' => {'molecule_idref' => '2'}
        };