well if you still want to use a while loop then make it abit better
to show the channels the bot is in

on $*:TEXT:/^!chanlist$/iS:#: {
  var %a = $chan(0)
  while (%a) {
    var %m $addtok(%m,$chan(%a),32)
    dec %a
  }
  msg # I'm in: %m
}

and $comchan so it would show the channels which both you and the bot are on.

on $*:TEXT:/^!chanlist$/iS:#: {
  var %a = $comchan($nick,0)
  while (%a) {
    var %m $addtok(%m,$comchan($nick,%a),32)
    dec %a
  }
  msg # Me and $nick are in these same channels: %m
}

but i would still go the $regsubex way as i stated before