#!/usr/bin/perl
use strict; use warnings; use XML::Twig;
my $xml=join('', <DATA>);

my $max_docid=0;
XML::Twig->new(
    twig_roots =>{
        'str[@name="docuid"]' => \&set_max_docid,
    },
)->parse($xml);

sub set_max_docid {
    my ($twig, $elt)= @_;
    my $docid= $elt->first_child_trimmed_text;
    if ($docid > $max_docid ){
        $max_docid=$docid
    }
}

print "max docid=$max_docid\n";
__DATA__
<all>
<doc>
<date name="processingtime">2011-04-09T11:12:22.049Z</date>
<str name="docuid">121422</str>
<str name="title">ABC</str>
</doc>
<doc>
<date name="processingtime">2012-04-09T11:12:22.049Z</date>
<str name="docuid">13427</str>
<str name="title">CDE</str>
</doc>
<doc>
<date name="processingtime">2010-04-09T11:12:22.049Z</date>
<str name="docuid">89822</str>
<str name="title">LKK</str>
</doc>
</all>