using tye's reference approach without patching the sources:

You could use actions to set and unset a global variable holding a ref to the variable to be watched:

something like the following watches between the lines 10 and 13 only!

  DB<2> a 10 $WATCH=\$t
  DB<3> a 13 $WATCH=\undef
  DB<4> w ${$WATCH}
[download]

IMHO you could even define your own alias in .perldb to automate this.

something like rw 10-13 $t for "regional watch".

Tye's remarks about weakening to avoid leaks are still valid...