lookbehind regexp

cmic has asked for the wisdom of the Perl Monks concerning the following question:

Was trying to pring only lines *not* begining with TAGS, but this snippet doesn't works. And I don't know why.. Thans for any explanation.

#
# Negative lookbehind , huh ??
#
while (<DATA>) {

    if (/(?<!TAGS)(.*?)(?=TAG2)/xgi ) {
        print "Negative lookbehind: ";
        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
[download]

This code prints:
Negative lookbehind: TAG1 text one
Negative lookbehind: TAG1 text two
Negative lookbehind: TAG1 text three
Negative lookbehind: TAGS text four
Negative lookbehind: TAG1 text six

-- cmic. Life helps. Perl Too.

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