$seen =~ s/'/''/g;
$interpolated = $seen? "'$seen'" : 'NULL';
$dbh->do("INSERT INTO foo (bar) VALUES ($interpolated)");

##</code><code>##

my $sth = $dbh->prepare('INSERT INTO foo (bar) VALUES (?)');
$sth->execute($seen);

##</code><code>##

$sth->execute( map {$_ ? $_ : undef} @seen_values );