It seems you want to write a pattern that says "match PAT1, and there should not be PAT2 somewhere after it". You can write that as: 
/PAT1(?!.*PAT2)/s

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or, in your case, as 
$string =~ /^ERROR(?!.*\b(?:Error processing Cancel Execution|license 
+will expire)\b)/ms

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

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