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my $valid_chars = 'CETLMNPA';
if ($CD =~ m/^[$valid_chars](?:,[$valid_chars])*$/) {
...
else {
   die "Invalid!"
}

m/
   ^[$valid_chars]   #the string must start with one of the valid char
+s
...
   )*                #and that comma+valid char could repeat 0 or more
+ times
   $                 #until the end of the string
/x                   #this is here to allow me to use all these commen
+ts

my %config = map { $_ => 1 } split(',', $CD);
## later on...
if ( $config{L} ) { print "L was set!" }