my $errmsg = '';
my $sql = "INSERT INTO url (url, url_id) VALUES (?, ?)";
my @bind = ( 'blah.com', 3 );
$dbh->do($sql,{},@bind);
if( $dbh->err ){
  warn "INSERT url FAILED: " . $dbh->errstr;
  $errmsg = "There was an error adding the URL";
}

##</code><code>##

if( $dbh->err ){
  warn "INSERT url FAILED: " . $dbh->errstr;
  if( $dbh->err eq '123' ){
    $errmsg = "Please fix ___ before adding the URL.";
  }elsif( $dbh->errstr =~ /not unique/ ){
    $errmsg = "This URL already exists.";
  }else{
    $errmsg = "There was an error adding the URL";
  }
}

##</code><code>##

my $tmp_id = $dbh->selectrow_array("select url_id from url where url = ?",{},$url);
if( $ct ){
  $errmsg = "The URL '$url' already exists w/id=$tmp_id.";
  return;
}
...
INSERT INTO ...