That's going to be tricky for many reasons.
First of all, any kind of command line script or tool running from a LaunchAgent is not running in Terminal. Terminal's not involved at all.
External volumes are a separate protection category. Normally, to access those permissions, an app must request them and that triggers a system dialog box. But only a full app can trigger this. Command line tool simply fail. You'll need to manually provide Full Disk Access. I'm not 100% sure what you need to give full disk access to. You may need to give it to the Perl executable itself. If your Perl script is executable, it's possible you could give access just to the script.
In the past, the answer would have been to give Full Disk Access to the Perl executable. But I haven't tried it in Tahoe and Apple likes to change/break these things. See if you can give Full Disk Access to the script, if it's executable. If not, try the Perl executable. Also try your script in an internal folder that isn't protected. These days, that might be only /tmp.
I'm sorry I don't have more definitive information. Modern iOS simply isn't very amenable to command line tools. It's designed for stand-alone, fully sandboxed and Apple-provisioned apps. Anything else is going to be a challenge. And it could fail at any time, without notice. Apple is under intense hacking pressure from its largest competitors. People running Perl scripts in Launch Agents just don't make the news in the same way.
