Replying from the front page I have not seen other replies.
To having answer one must remember that

• regexps are greedy
• \w matches _
• regexps do backtrack to get a match

'__xxx__' =~
  m/
     \b     # word boundary, matches begin of the string
     _+     # matches a couple of '_' because greedy 
     (\w+)  # matches the rest(greedy) but must do backtrack  for whol
+e regexp match
     _+     # after one backtrack matches last '_' 
     \b
  /x;
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