There isn't a simple way to do it. Your closest approach would be a custom C function to do the concatenation:
LET label_3 = CONCATENATE(sample.col1, sample.col2)
That, of course, relies on you having a custom Perform runner with a concatenate function added to it.
Perform pre-dates the addition of the '||' string concatenation operator into SQL and does not support it.
The alternative is to use an Informix 4GL (I4GL) program instead. You can do a lot of things in I4GL that you cannot do in ISQL - at the cost of writing the code.