... unintentionally modifying the hash values before the assignment [is] harmless in this case, but I think it's a bad habit.

The modification was certainly not unintentional (or unconsidered), but I take your point. I wish I had thought to include a cautionary note about this (in fact, I think I will right now!) when I was going on about key collision, as I wish I had thought to use the  apply function.

... loads of useless options on your substitution.

I have adopted this and other regex-related BPs from Conway's PBP. I know many monks would disagree, but the arguments for these practices are compelling to me.