This is because you did not understood the regular expressions at the fundamental level.

.*?
zero chars -> true -> after the current position is not (?!....) -> true -> return true

Use something like:
my $string = "ERROR blah license will expire";
if ($string =~ /^ERROR\b.+?(?:Error processing Cancel Execution|licens
+e will expire)\b/m) {
    print "NOT MATCHED\n";
}
else {
    print "MATCHED\n";
}

[download]
or:
if ($string =~ /^ERROR\b/m and $string !~ /\b(?:Error processing Cance
+l Execution|license will expire)\b/) {
    print "MATCHED\n";
}
else {
    print "NOT MATCHED\n";
}

[download]

In reply to Re: Regex: negative look ahead by Anonymous Monk
in thread Regex: negative look ahead by fcaw1

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