while (<DATA>) {

    if (/^(?!TAGS)(.*?)(?=TAG2)/xgi ) {
        print "Negative lookahead: ";
        print $1, "\n";
    }
}
#
__DATA__
TAG1 text one TAG2
TAG1 text two TAG2
TAG1 text three  TAG2
TAGS text four   TAG2
TAG1 text five TAGT
TAG1 text six TAG2