use DBI qw(SQL_VARCHAR);

$sth->bind_param(1, $mrn, SQL_VARCHAR);
$sth->execute();

##</code><code>##

     If any arguments are given, then "execute" will effectively
     call "bind_param" for each value before executing the
     statement.  Values bound in this way are usually treated as
     "SQL_VARCHAR" types unless the driver can determine the
     correct type (which is rare), or unless "bind_param" (or
     "bind_param_inout") has already been used to specify the
     type.