sub get_by_id {
    my ($self, $feed_id) = @_;
    my $feed = $self->single({id => $feed_id});
    sub XML::Feed::Content::body_alt { shift->_var('body_alt', @_) };
    my $xml_feed = XML::Feed->parse(URI->new($feed->url));
    $xml_feed->{'url'} = $feed->url;
    
    use WWW::Translate::Apertium;
    foreach ($xml_feed->entries) {
     my $engine = WWW::Translate::Apertium->new(lang_pair => 'en-ca');
     my $translated_string = $engine->translate($_->content->body);
        
     #This change $_->content->body_alt 
     $_->content->body_alt($translated_string);
    }
    
    return $xml_feed;    
}
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[% FOREACH entry IN feed.entries %]
    [% USE Dumper %]
    [% Dumper.dump(entry.content) %]
[% END %]
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$VAR1 = bless( { 'body' => 'This is a test post', 'base' => undef, 'ty
+pe' => 'text/html' }, 'XML::Feed::Content' );
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It seems that ->entries does not return a premade list of entries for you to modify. It always returns a fresh list of entries, so your modifications are gone on your second call.

I suggest that you ditch your idea of modifying XML::Feed and adding something to it, and switch to plain hashes, or alternatively add the method in question to XML::Feed::Content and call it from your template instead.

Monkey patch means extend/modify code while running / at run time. So the suggestion means, just add that code to your actual script/program and not your locally installed module. Looks like a good suggestion for the short term. Might submit a patch/ticket to the module to get the feature for real down the road.