use constant SELECT_USER_QUERY => q(
    SELECT userid, username, password
    FROM user_table,
    WHERE userid=?
);

#...much later...

my $sth = $dbh->prepare( SELECT_USER_QUERY ) or die DBI::errstr;
$sth->execute( $userid ) or die DBI::errstr;
my @data = $sth->fetchrow_array( $sth );