If all you want from the document is the maximal docuid, you can set the rules to give you exactly that:

#!/usr/bin/perl
use strict;
use warnings;
no warnings qw(uninitialized);

use XML::Rules;

my $xml = <<'XML';
<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>
XML


my @rules = (
    'str' => sub {
        return unless $_[1]->{name} eq 'docuid';
        my $id = $_[1]->{_content};
        $_[4]->{pad} = $id if ($id > $_[4]->{pad});
        return;
    },
    'all' => sub { return $_[4]->{pad}; }
);

my $parser = XML::Rules->new(rules => \@rules);
my $max_value = $parser->parse( $xml );

print "The max value is: $max_value\n";
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This assumes that you want the maximal value from any <str> tag with attribute name="docuid" as it doesn't check the "path" to the <str> tag!

Update: With version 1.16 and later it's easy to give the specific parser a more readable interface:

use XML::Rules max_docuid => {
  method => 'parse',
  rules => {
    'str' => sub {
      return unless $_[1]->{name} eq 'docuid';
      my $id = $_[1]->{_content};
      $_[4]->{pad} = $id if ($id > $_[4]->{pad});
      return;
    },
    'all' => sub { return $_[4]->{pad}; }
  }
};
#...
print "The max value is: " . max_docuid($xml) . "\n";
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