> "oXoXoXoX" =~ /(?:.X(?{return "Yes n=$n" if ++$n==$m}))*/

looks like a very complicated way of trying

  DB<44> sub func { my ($m)=@_; return "Yes n=$m" if 'oXoXoXoX' =~ /(?
+:.X){$m}/ }

  DB<45> p func(0)
Yes n=0
  DB<46> p func(2)
Yes n=2
  DB<47> p func(4)
Yes n=4
  DB<48> p func(5)

  DB<49>
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