my $string = "Hello";
if ($enc) {
   print '{blowfish}', encrypt_string ($string);
} else {
   print "{unencrypted}$string";
}
....
my $val = $q->param ('foo');
my $actual_val;
if ($val =~ /^{([a-z]+)}(.*)$/) {
   if ($1 eq 'blowfish') {
      $actual_val = decrypt ($2);
   } else {
      $actual_val = $2;
} else {
   $actual_val = $val;   # didn't match - old-style param?
}