Or, instead of grabbing what you want, just get rid of (or substitute) what you don't want (don't neglect the space after (Unknown) if you do want such an attribution):

while(<FILE>){
    $_ .= '-- (Unknown) ' unless /--/;
    s/^\s+//;
    s/\s+$/\n/;
    s/\s*--\s*/\t/;
    print;
}
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