I prefer use variable instead of writing giveawayword to txt file.
Code:
on *:TEXT:!giveaway start *:#:{
  if ($read(sreglist.txt,nw,$nick)) && ($3 != $null) {
    if (%giveaway == off) || (%giveaway == $null) {
      set -e %giveaway on
      msg $chan giveaway started. Clearing old giveaway list. Please say " $+ $3 $+ " to enter the giveaway!
      write -c giveaway.txt
      set -e %giveawayword $3
    }
    elseif (%giveaway == on) msg $chan a giveaway is still on. please end it by saying "!giveaway end"		 
  }
}

on *:text:%giveawayword:#:{
  if (%giveaway == on) && !$read(giveaway.txt,nw,$nick)) {
    write giveaway.txt $nick
  }
}