APPARENT SOLUTION

This appears to be the only way to resolve this on Ubuntu 22.04 LTS (Jammy Jellyfish). The Repositories for Ubuntu 20.04 (Focal Fossa) must be added to the sources.list -- at least, this is what enabled me to install a "working" version of zef (which actually seems broken in terms of actually installing anything, due to failures on module repositories/dependencies, etc.).

Start by navigating to the /etc/apt directory, then edit the sources.list file to include the following two lines:

deb http://archive.ubuntu.com/ubuntu/ focal main restricted universe m
+ultiverse
deb-src http://archive.ubuntu.com/ubuntu/ focal main restricted univer
+se multiverse
[download]

Next, run:

apt update
[download]

After which an "apt-cache search zef" will return the perl6-zef module candidate (perl6-zef - Perl 6 package manager). Next:

apt-get install perl6-zef
[download]

...and you should be in business--at least, I was...to the following results:

apt-get install perl6-zef
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following NEW packages will be installed:
  perl6-zef
0 upgraded, 1 newly installed, 0 to remove and 100 not upgraded.
Need to get 402 kB of archives.
After this operation, 2,564 kB of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu focal/universe amd64 perl6-zef 
+all 0.8.2-1 [402 kB]
Fetched 402 kB in 2s (232 kB/s)     
Selecting previously unselected package perl6-zef.
(Reading database ... 417807 files and directories currently installed
+.)
Preparing to unpack .../perl6-zef_0.8.2-1_all.deb ...
Unpacking perl6-zef (0.8.2-1) ...
Setting up perl6-zef (0.8.2-1) ...
Scanning processes...                                                 
+                                                                     
+                                        
Scanning candidates...                                                
+                                                                     
+                                        
Scanning linux images...                                              
+                                                                     
+                                        

Restarting services...
[download]

Apparently, however, there is no manual for zef.

# man zef
No manual entry for zef
[download]

...and it does not seem to have a complete list of Raku modules.

# zef install DBIish
===> Searching for: DBIish
===> Updating cpan mirror: https://raw.githubusercontent.com/ugexe/Per
+l6-ecosystems/master/cpan1.json
===> Updated cpan mirror: https://raw.githubusercontent.com/ugexe/Perl
+6-ecosystems/master/cpan1.json
===> Updating p6c mirror: https://raw.githubusercontent.com/ugexe/Perl
+6-ecosystems/master/p6c1.json
===> Updated p6c mirror: https://raw.githubusercontent.com/ugexe/Perl6
+-ecosystems/master/p6c1.json
No candidates found matching identity: DBIish
[download]

So...I still cannot install the one module I most need at present: DBIish.

I may be back to Perl5 simply because Raku is too broken to use. :(