#!/usr/bin/perl

use strict;
use warnings FATAL => "all";
use XML::Simple;

my $xml =
'<pages>
<contentmodel>
<level>5</level>
<first>35</first>
<content>some text</content>
</contentmodel>
<contentmodel>
<level>4</level>
<first>45</first>
<content>some text</content>
</contentmodel>
<contentmodel>
<level>3</level>
<first>25</first>
<content>maps</content>
</contentmodel>
<contentmodel>
<level>2</level>
<first>15</first>
<content>some text</content>
</contentmodel>
</pages>
';

my $ref = XMLin($xml);

for my $cm(@{$ref->{'contentmodel'}}){
     printf "<first>%d</first>\n", $cm->{'first'}
        if $cm->{'content'} eq 'maps'
};

__END__

output

<first>25</first>