And after some digging, here's a systemd version. It uses the very same server code in /home/rick/code/pm11157093.pl from this node's parent (change to your liking), and is called pfs (for perl forking server).

Make file /etc/systemd/system/pfs.socket

# perl forking server https://perlmonks.org/?node_id=11157093
[Unit]
Description=perl forking socket

[Socket]
ListenStream=1234
Accept=yes
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and file /etc/systemd/system/pfs@.service

# perl forking server https://perlmonks.org/?node_id=11157093
[Unit]
Description=perl forking server
Requires=pfs.socket

[Service]
ExecStart=-/home/rick/code/pm11157093.pl
StandardInput=socket
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run systemctl start pfs.socket and fire up a telnet to test it.

Some extra config lines may be required to start automatically at boot, plus systemctl enable pfs.socket

Perhaps a systemd expert can fill in any holes I've missed.

UPDATE: the remote addr is in $ENV{REMOTE_ADDR} (see man systemd.socket)