I usually use XML::LibXML to handle XML.

#!/usr/bin/perl
use strict;
use warnings;

use XML::LibXML;

my $item_list = shift;

my $xml = << '__XML__';
<BulkUpdate>
  <UpdateSets>
    <UpdateSet>
       <type name="MyType" />
       <where>
         <cond_prop attrName="ID" cond_operator="="/>
       </where>
       <update>
          <update_prop attrName="sh4_SAP_ID" attrValue="" />
       </update>
    </UpdateSet>
  </UpdateSets>
</BulkUpdate>
__XML__

my $dom = 'XML::LibXML'->load_xml(string => $xml);
my ($sets) = $dom->findnodes('/BulkUpdate/UpdateSets');
my ($update) = $sets->findnodes('UpdateSet');

open my $in, '<', $item_list or die $!;
while (<$in>) {
    chomp;
    my $set;
    if ($. > 1) {
        $set = $update->cloneNode(1);
        $sets->appendChild($set);
    } else {
        ($set) = $dom->findnodes('/BulkUpdate/UpdateSets/UpdateSet[las
+t()]');
    }
    my ($cond_prop) = $set->findnodes('where/cond_prop');
    $cond_prop->{attrValue} = $_;
}
print $dom;
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