use strict;
use warnings;
use XML::LibXML;

my $doc = XML::LibXML->new->parse_fh(\*DATA);

for my $node ( $doc->findnodes('//experiment[@accnum]') ){
    print $node->getAttribute("accnum"), "\n";
}

__DATA__
<?xml version="1.0" encoding="UTF-8"?>
<experiments total-hybs="629" total-samples="733" total="21" to="21" from="1">
  <experiment accnum="E-GEOD-8918" array="Agilent Human Genome CGH Microarray 44B [G4410B]" biosamples-png="download/mageml/E-GEOD-8918.biosamples.png" biosamples-svg="download/mageml/E-GEOD-8918.biosamples.svg" fgem="download/mageml/E-GEOD-8918.processed.zip" fgem-count="87" hybs="87" id="1636341195" name="CGH profiling of 87 indolent non-hodgkin s lymphoma (NHL)" releasedate="2008-06-19" samples="174" sdrf="download/mageml/E-GEOD-8918.sdrf.txt" species="Homo sapiens" two-columns="download/mageml/E-GEOD-8918.2columns.txt">
    <users>
      <user id="1"/>
    </users>
    <secondaryaccessions>
      <secondaryaccession>GSE8918</secondaryaccession>
    </secondaryaccessions>
  </experiment>
</experiments>