my $in = "a,b";
my $sth = $dbh->prepare("SELECT foo FROM bar WHERE baz IN (?)");
$sth->execute( $in );

# fetch returns no results -- it would work if $in were just "a"


my @in = qw/a b/;
$sth = $dbh->prepare("SELECT foo FROM bar WHERE baz in (?,?)");
$sth->execute( @in );

# fetch returns expected results for "a" and "b"