Well, aside from using CGI three things: you should move the call to exit out of all the different subs and place it after your dispatch code (repeated code is bad); you're not locking the database file (perldoc -f flock); and your dispatch code might be a little cleaner this way:

my %actions = (
               Add    => \&Add,
               Delete => \&Delete,
               Show   => \&Show,
               ...
              );

if( length( $action ) ) {
  $actions{ $action }->( );
} else {
  Show();
}

exit 0;
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That way you just have to maintain the dispatch table in the %actions hash rather than having n different if statements (repeated code bad, again :).