local $"=",";
my $sth = $dbh->prepare(
       "SELECT * FROM $table 
        WHERE lastname  = $lastname 
        AND   firstname IN (@firstnames)
       "
);
$sth->execute( $lastname, @firstnames );

##</code><code>##

my $sth = $dbh->xprepare( ph {
       "SELECT * FROM $_table 
        WHERE lastname  = $lastname 
        AND   firstname IN (@firstnames)
       "
});
$sth->xexecute();