As the script is on Debian stable it's not the most up-to-date version of FB - which is at v3.05. I couldn't see anything in the active or resolved bug lists that appeared to relate to this.

I explicitly set action => $ENV{SCRIPT_NAME}, and that returns action="/cgi/<scriptname> in the form.

I have now updated CGI.pm and CGI::FormBuilder and the problem seems to still exist when using a default undeclared action at the later versions.

UpdateMy Perl isn't good enough to understand FormBuilder.pm yet but I've filed a bug with the workaround you suggested

Thank you for your help