# @ins is actually calculated at runtime. 
# Defined here for demonstration.
my @ins = qw(1 2 3 4 5 6);
my $in = join ', ', @ins;
my $sth = $dbh->prepare("SELECT foo FROM Bar WHERE baz IN ( ? )");
$sth->execute( $in );

##</code><code>##

my @ins = qw(1 2 3 4 5 6);
my $in;  $in .= '?, ' for @ins;

# discard hanging comma
$in =~ s/, $//;
my $sth = $dbh->prepare("SELECT foo FROM Bar WHERE baz in ( $in )");
$sth->execute( @ins );