$dbh2->do("INSERT INTO PerlTest (Postcode,street_column,streetCount) VALUES (?,?,?)", undef, '', $mystreet, 0);

##</code><code>##

$dbh2->do("INSERT INTO PerlTest (Postcode,street_column,streetCount) VALUES ('',?,0)", undef, $mystreet);

##</code><code>##

my $sth = $dbh2->prepare("INSERT INTO PerlTest (Postcode,street_column,streetCount) VALUES (?,?,?)");
...
$sth->execute('', $mystreet, 0);

##</code><code>##

my $sth = $dbh2->prepare("INSERT INTO PerlTest (Postcode,street_column,streetCount) VALUES ('',?,0)");
...
$sth->execute($mystreet);