Just a quick update. I've completed the upgrade: all went well.

No problems in the Perl space. I successful ran a few scripts. My GUIs (Tk) worked as expected. As a test for cpan, I installed Acme: all good. No problems with Perlbrew. The OS has its own, two versions of Perl.

$ /usr/bin/perl -v | head -2 | tail -1
This is perl 5, version 18, subversion 2 (v5.18.2) built for darwin-th
+read-multi-2level
$ /usr/local/bin/perl -v | head -2 | tail -1
This is perl 5, version 18, subversion 1 (v5.18.1) built for darwin-th
+read-multi-2level
$ which perl
/Users/ken/perl5/perlbrew/perls/perl-5.24.0t/bin/perl
$ perl -v | head -2 | tail -1
This is perl 5, version 24, subversion 0 (v5.24.0) built for darwin-th
+read-multi-2level
$ perlbrew switch perl-5.25.9t
$ which perl
/Users/ken/perl5/perlbrew/perls/perl-5.25.9t/bin/perl
$ perl -v | head -2 | tail -1
This is perl 5, version 25, subversion 9 (v5.25.9) built for darwin-th
+read-multi-2level
[download]

Not related to Perl, but just for general information: it's ~5Gb download; the subsequent installation took maybe an hour (I wasn't watching it that closely); all of my standard applications (including Firefox, Thunderbird, VLC, OpenOffice, Foxit) worked as previously (i.e. no reinstallation or reconfiguration required); the overall look-and-feel is virtually identical.