interesting... removing the "." command prefix has no effect whatsoever on the result -- the following snippet does NOT work as expected:

;;-- triggered for incoming DCC chat requests
;;
on ^*:OPEN:=:{
echo on OPEN $1-
dcc reject
close -ic $nick
halt
}

...that is, NO echo of "on OPEN" no obvious result from "dcc reject" and the DCC chat window remains open.

Also, curiously, this snippet does work as I'd expected:

on *:INVITE:#:{
.echo on INVITE # $1
describe # has been invited to #
halt
}

...that is, on receiving an INVITE, the following is displayed:
-- in the client status window --
on INVITE <channel-name>
-- in the active channel window of the client and remote --
<nickname> has been invited to <channel-name>

and I'm using mIRC 6.35