yes but thats not really right. Meaning you can't

switch ($left($1,1)) {
case a:
echo -a it's an a
default:
echo -a it's not an a
}

You would have to assign the return value of $left to a variable, which makes it messy. Also it gets even more messy if you want to use something with spaces because instead of:

case "this is a test"

You need to
:this $+ is $+ a $+ test

But afaik, that doesn't even work, because mIRC doesn't evaluate an identifier in a label, so that would simply translate to ":this" which most likely, is not what the scripter would have intended.