Oh, I think I find the answer. The problem should be with the string, in the first post the string should cause no problem

$sentence = " he PP
               study VB
               A~~~language~A NP
               and CC
               play VB
               A~~~game~A NP
               ."
[download]

but if I change the string into this

$sentence = " he PP
               study VB
               A~~~language~A NP
               and CC
               play VB
               A~~~(G|g)ames?~A NP
               ."
[download]

because /(G|g)ames?/ matches Games, Game, games, game, but it will not match "(G|g)ames?". I think that's the reason.

but how can I sub the $_ from $remainPart anyway?