next time please condense it to the relevant part!

> Seems a bit weird to have the end of a local sub clear '$1', yet that seems to be what is happening

yes, easily shown in a SSCCE

  DB<3> sub bla { "XXX"=~/(X*)/; print "inside $1" }

  DB<4> bla; print "outside $1"
inside XXXoutside
  DB<5>
[download]

> What was the logic of forcing/doing that?

I'd say it's about localizing the inner sub to protect all caller levels from effects at a distance, consider

  DB<5> "YYY"=~/(Y*)/; bla; print "old $1"
inside XXXold YYY
  DB<6>
[download]

otherwise nobody could rely on $1 etc anymore after calling a random sub.

Using a dedicated closure var holding the copied content of $1 is the way to go in your use case.