Can I use cpan inside a perlbrew context -- to install perl modules to a particular perlbrew perl install, only.

Yes.

perlbrew use NAME
cpan MODULE
[download]

or

"${PERLBREW_ROOT:-$HOME/perl5/perlbrew}/perls/NAME/bin/cpan" MODULE
[download]

(Replace NAME with the name of the perlbrew installation you want to use, and MODULE with the name of the module you want to install.)

Do I prefix the cpan install command with sudo? As in sudo perl -MCPAN -e'install Blah::blah'?

Not likely. The default settings for perlbrew places the Perl installations in your own home directory. It's unlikely that someone else installed a Perl there, so it wouldn't make sense to pretend to be someone else.

Can I actually use cpan to install modules in a particular perlbrew perl install only? Is that syntax without the sudo?

Seems you asked the same question twice?