I guess there'll be quite a few answers to this but here's my attempt:

$newtext = "some new text";
$spantext =~ s/(\<span[^>]+\>)[^<]+(<.*>)/$1${newtext}$2/i;
#assuming there's no tags between the spans...
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rdfield

update
running

$_='7065726c646f63207065726c7265';system pack"H*",$_
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might help too :)