That's more complicated than I thought because of the special nature of tr's arguments. :(

double-quotes allow interpolation and escape codes, single-quotes don't

  DB<76> p "a\bc"
c
  DB<77> p length "a\bc"
3
  DB<78> p 'a\bc'
a\bc
  DB<79> p length 'a\bc'
4
  DB<80> p 'a\b' eq 'a\\b'
1
[download]

But tr forbids interpolation while allowing escape codes

  DB<80> say '$l' =~ tr/$l/xyz/r    # no interpolation
xy

  DB<81> say "a\b" =~ tr/a\b/xyz/r  # but escape codes
xy
[download]

This makes evaling tr in a predictable way far too complicated.

Just escaping delimiters won't do, if escape codes are involved.

I think it's wise to give it up here.