If you're checking logins against a user table, you could count the number of rows that match your u/p combination

my $sql = q{
   select count(*) 
   from app_users 
   where username = ? and 
         password = ?
};

if (($dbh->selectcol_arrayref($sql, undef, $username, $password)->[0] 
+== 1) {
   # We've logged in
} else {
   # login error
}
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Of course, you probably shouldn't store your passwords in plain text. Assuming you've got some sort of one way encryption going on (using Perl's crypt, or a MD5 or SHA module), you could simply replace the test with

if (($dbh->selectcol_arrayref($sql, undef, $username, my_crypt_call($p
+assword))->[0] == 1)
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By the way, don't interpolate variables in your SQL, or you'll make your login susceptible to a SQL injection attack. Always use placeholders

Update: corrected syntax