just use variables without {{}} in an assignment

var_a := "hello"
var_b := "world"

var_v1 := var_a+"-"+var_b
var_v2 := 'export exp_a='+var_a+'; echo "${exp_a}-'+var_b+'"'
var_v3 := var_a + var_b

@default:
  echo var_a: {{var_a}}
  echo var_b: {{var_b}}
  echo var_v1: {{var_v1}}
  echo var_v3: {{var_v3}}
  #echo var_v2: {{var_v2}} #what are you trying to do?