If the goal here is to do things with strings which do not match a pattern, don't try to use a negative look-ahead -- just negate the test:

my @strings = (
    "ERROR blah license will expire",
    "ERROR blah something else",
    "ERROR blah Error processing Cancel Execution",
);
foreach (@strings) {
    if (!($_ =~ /^ERROR.+?(Error processing Cancel Execution|license w
+ill expire)$/)) {
      print "Do something with '$_'\n";
    }
}

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

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