use CGI qw(:cgi);
use DBI;

# sanitize() not shown
my $username = sanitize(param('username'));

my $dbh->connect(assuming_relevant_connection_params_are_here);

my $sql = qq{ SELECT COUNT(*) FROM profiles WHERE username=? };

my $result = $sth->execute($username)
  or bail_out('Error executing SELECT');

$dbh->disconnect;

# do something based on $result

sub bail_out {
  my $msg = shift;
  # prints error in html
  exit(0);
}