perlbrew off has no corresponding perlbrew on

[matthewpersico]

It would be nice to replace this workflow:

export PERLBREW_VERSION_LAST=$PERLBREW_VERSION
perlbrew off
## do some stuff with system perl
perlbrew use $PERLBREW_VERSION_LAST

with

perlbrew off
## do some stuff with system perl
perlbrew on

and have perlbrew use remember what your last version was. I have code that I have to tidy up and will present as a PR to try this in the next few days.

[guest20]

Also, be sure to make a The IT Crowd reference in your updated docs