From "perldoc perlop":

Binary "!~" is just like "=~" except the return value is negated in th
+e
       logical sense.
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So - use your pattern, and negate the match.

Try this pattern:

!~/(:?[\da-f]{2}:){5}[\da-f]{2}/
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or better, Regexp::Common::net provides built-in regex matching for mac addresses.