my $p;
my ($userid,$firstname);
my $sth = $dbh->prepare(q{Select user_id,firsname from user where user_id = ?});
$sth->execute( $user_name );

while( $p = $sth->fetchrow_hashref )
{
   $userid = $p->{user_id};
   $firstname = $p->{firstname};
}

if(defined $username)
{
   print "Hello $firstname, welcome back!";
}
else
{
   # display an error message, code somewhere else
   loginerror($userid);
}