Hi, Jim -
It looks like somebody was attempting to emulate a C STRUCT data format definition.
I couldn't find anything in the man page for ksh that came with my FreeBSD-5.4 pdksh distro that looks anything like what you describe.
Check first for a macro definition that generates the construct. It'll probably be a string mapping construct.
I would seriously doubt that your Perl version needs this for either ASCII or binary transfers. Net::FTP away! :D