O.K. I have modified my regexp in the following way (I want to focus first on less detail to see what's happening):

$MType =~ s/\s+//g;
            
if ($MType =~ /^(?:-41)$/i ||
    $MType =~ /^(?:-55)$/i ||
    $MType =~ /^(?:-jj)$/i) {
     print "\"red\"";
} else {    
     print "\"#CCFFCC\"";
}
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This however, does not return any results whatsoever now. I split it into three individual componets because I wanted to see if the engine will match that way. No go.