I'm used to use XML::LibXML and XPath

use strict;
use warnings;
use XML::LibXML;

my $doc         = XML::LibXML->new->parse_file('abc.xml');
my @class_elems = $doc->findnodes('/*/class');
my @sorted_class_elems
    = sort { $a->findvalue('strength') <=> $b->findvalue('strength') }
    @class_elems;
for my $node (@class_elems) {
    $node->replaceNode( shift(@sorted_class_elems)->cloneNode(1) );
}

open( my $fh, '>', 'xyz.xml' );
$fh->print( $doc->toString(1) );
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Roman