#!/bin/bash
s='    some text
    <addresses>
      <something/>
    </addresses>
    some more text
    <addresses xmlns="namespace">
      <could be anything/>
    </addresses>
    some other text
    <addresses>
      <something else/>
    </addresses>
    ...'
ad='addresses'
nm='namespace'
perl -0777 -pe 's~<('"$ad"')\s+xmlns="'"$nm"'">[^<]*(?:<(?!/\1>)[^<]*)*\K</\1>~</'"$nm"':$1>~' <<< "$s"