What do you mean delete? Permanently remove, or remove for the current instance/session? I ask because in the context of CGI this sounds a little odd. An alternative to a flat file would be a DBM. With DBM I would start with the values set to 1, and en lieu of deleting the key set the value to 0; that ought to be very IO (UPDATE: and memory) efficient.

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perl -pew "s/\b;([mnst])/'$1/g"