$ cat go.awk
BEGIN {
  print "The BEGIN block is executing"
  print reverse("The BEGIN block is executing")
}

{
  print reverse($0)
}

function reverse(str) {
  trs=""
  for(i=length(str); i > 0; i--) {
    trs=trs substr(str, i, 1);
  }
  return trs
}

$ cat input
line one
line two
line three

$ awk -f go.awk < input
The BEGIN block is executing
gnitucexe si kcolb NIGEB ehT
eno enil
owt enil
eerht enil