format

format (tf.io)

format("%s-%s", var.your-first-string, var.your-second-string)
PROD-Value-of-your-second-string

provider "aws" {
  region = "eu-central-1"
  assume_role {
    role_arn = format("arn:aws:iam::0987654321:role/your-role-%s", var.youraction)
  }
}

See also

• Terraform functions, Terraform collection functions: tolist, toset, length, sum, file, join, depends_on, zipmap, replace, lookup, read files, concat, merge, templatefile, for_each, format, element, slice, try, filebase64, upper, filemd5, coalesce, formatlist, flatten, sensitive, tonumber, jsondecode, yamldecode, pathexpand