Code:
on ^*:TABCOMP:*:{
  echo -a ^ Event triggered
}

on *:TABCOMP:*: {
  echo -a Regular event triggered
}


Case 1: I type something that will NOT be completed by mIRC:
Regular event triggered

Case 2: I type something that WILL be completed by mIRC:
^ Event triggered
Regular event triggered

Code:
on ^*:TABCOMP:*:{
  echo -a ^ Event triggered
  haltdef
}

on *:TABCOMP:*: {
  echo -a Regular event triggered
}


Case 1: I type something that will NOT be completed by mIRC:
Regular event triggered

Case 2: I type something that WILL be completed by mIRC:
^ Event triggered