Use XML::Twig to process the xml, and List::Util to assist you in finding the min/max/sum/average.

use List::Util;
use XML::Twig;

use strict;
use warnings;

my $xml = do {local $/; <DATA>};

my $twig = XML::Twig->new;
$twig->parse($xml);

my @lts = map {$_->att("lt")} $twig->findnodes(q{//httpSample[@lt]});

print join(',', @lts), "\n";

print min(@lts), "\n";
print max(@lts), "\n";
print sum(@lts), "\n";
# ... pretty easy from ehre

__DATA__
<?xml version="1.0" encoding="UTF-8"?>
<testResults version="1.2">
<httpSample t="704" lt="704" ts="1306146504248" s="true" lb="HTTP Requ
+est" rc="200" rm="OK" tn="Thread Group 2-1" dt="text" by="411"/>
<httpSample t="525" lt="525" ts="1306146505234" s="true" lb="HTTP Requ
+est" rc="200" rm="OK" tn="Thread Group 2-2" dt="text" by="411"/>
<httpSample t="586" lt="586" ts="1306146611316" s="true" lb="HTTP Requ
+est" rc="200" rm="OK" tn="Thread Group 2-1" dt="text" by="411"/>
<httpSample t="523" lt="523" ts="1306146612307" s="true" lb="HTTP Requ
+est" rc="200" rm="OK" tn="Thread Group 2-2" dt="text" by="411"/>
<httpSample t="507" lt="507" ts="1306146613306" s="true" lb="HTTP Requ
+est" rc="200" rm="OK" tn="Thread Group 2-3" dt="text" by="411"/>
<httpSample t="535" lt="535" ts="1306146615306" s="true" lb="HTTP Requ
+est" rc="200" rm="OK" tn="Thread Group 2-5" dt="text" by="411"/>
<httpSample t="526" lt="526" ts="1306146506234" s="true" lb="HTTP Requ
+est" rc="200" rm="OK" tn="Thread Group 2-3" dt="text" by="411"/>
<httpSample t="499" lt="498" ts="1306146507234" s="true" lb="HTTP Requ
+est" rc="200" rm="OK" tn="Thread Group 2-4" dt="text" by="411"/>
<httpSample t="505" lt="505" ts="1306146508234" s="true" lb="HTTP Requ
+est" rc="200" rm="OK" tn="Thread Group 2-5" dt="text" by="411"/>
<httpSample t="536" lt="536" ts="1306146509249" s="true" lb="HTTP Requ
+est" rc="200" rm="OK" tn="Thread Group 2-6" dt="text" by="411"/>
</testResults>
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