If you want the "unique somethings", use a hash. Here you key off of the message ID and store the line of text as the value.

Code, untested --

my %messages;  

while(<FILE>) {
    /(FW\-\d+\-\d+)/;               # extract MSG ID
    next if exists($messages{$1};   # ignore it if we've
    $messages{$1} = $1;             # alreaqdy seen it.
}
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%messages has the first occurance of each message