As a matter of style, I prefer to use a hash to figure out whether something is in a set, or not. So your code would be simplified to the following:

sub triggerNotification {

  my ( $data ) = @_;

  my %message_classes = (
    'ARCHITECH' => 1,
    'CURR-LIB'  => 1,
    'MARKET_PL' => 1,
  );

  if ( not exists $message_classes{ $data } ) {

    #  Send an E-Mail notification if the message type is not listed.
  }
}
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Using 'data' has a variable name is amusing -- you can probably find a better name than that. :)