The problem lies in the "\b" at the start and end of your regex.

See:

$_ = "ab_";
print "\\b matches\n" if m!ab_\b!;
$_ = "ab*";
print "\\b matches\n" if m!ab\*\b!;
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Only the first matches.

So if you change your regex to:

   m@((\*|_)\S+\b)(.+?)(\b\S+\2)@s
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it seems to work.