SIP2 emulator not working on main - connection refused (is working on 23.11.x)

I can't seem to get the SIP2 emulator working on main - I get a connection refused error. It works as expected on 23.11.x.

This is with a fresh koha-testing-docker git clone and ktd --pull, with only .env changes for Bugzilla credentials.

I'm not sure whether it is a KTD issue or a Koha issue.

Results of trying a SIP2 command:

david@david-thinkpad-t14:~/git-koha/koha-testing-docker$ ktd --shell
kohadev-koha@kohadevbox:koha(23.11.x)$ ./misc/sip_cli_emulator.pl -a 127.0.0.1 -p 6001 -su term1 -sp term1   -l CPL --patron 2352900100046 -m fee_paid --fee-type FT --fee-amount 0.01
Attempting socket connection to 127.0.0.1:6001...connected!
Use of uninitialized value $hold_mode in string eq at ./misc/sip_cli_emulator.pl line 123.
SEND: 9300CNterm1|COterm1|CPCPL|
READ: 941

Trying 'fee_paid'
SEND: 3720240501    192211FT00USDBV0.01|AO|AA2352900100046|ACterm1|
Use of uninitialized value $data in concatenation (.) or string at ./misc/sip_cli_emulator.pl line 355, <GEN0> chunk 1.
READ: 
kohadev-koha@kohadevbox:koha(23.11.x)$ exit
exit
david@david-thinkpad-t14:~/git-koha/koha-testing-docker$ ktd --shell
kohadev-koha@kohadevbox:koha(main)$ ./misc/sip_cli_emulator.pl -a 127.0.0.1 -p 6001 -su term1 -sp term1   -l CPL --patron 2352900100046 -m fee_paid --fee-type FT --fee-amount 0.01
Attempting socket connection to 127.0.0.1:6001...failed! : Connection refused
kohadev-koha@kohadevbox:koha(main)$