Says ichimunki:

Since you're being incredibly helpful, can I impose on you further than anon_monk did?

Um, no. Now you're asking me to read you the manual. If you read it and you don't understand, then I will try to answer. You could also try writing a few small example programs to test out your guesses about how it works.

is there a way to insert a last/return clause into the callback which aborts the request?

The general pattern for such things in Perl is:

    eval { $ua->request($request, \&callback, 4096) };
    if ($@ =~ /^Request aborted/) {
      # handle aborted request
    } elsif ($@) {
      die;
    }
    
    sub callback {
      # ...
      die "Request aborted" if ...;
      # ...
    }
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