I like to use local $_ = $_; to prevent the arguments from being modified:

($sEnd) = map {uc}
          map {s/world/todo el mundo/; $_}
          map {local $_ = $_; s/hello/goodbye/; $_}
          $sBegin;
# Or:
($sEnd) = map {uc}
          map {s/world/todo el mundo/; $_}
          map {s/hello/goodbye/; $_}
          map {local $_ = $_}
          $sBegin;
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