#!/usr/local/bin/perl
#use strict;
use warnings;
#use re debug;

while (<DATA>) {
# This section works OK.
#   if (/(?<=TAG1)(.*)(?=TAG2).*?/x) {
#       print $1, "\n";
#   }

   if (/^(?<!TAGS)(.*?)(?=TAG2).*$/x ) {
       print $1, "\n";
   }
}

__DATA__
TAG1 text one TAG2 etc
TAG1 text two TAG2 etc
TAG1 text three  TAG2 etc
TAGS text four TAG2 etc
TAG1 text five TAGT etc