If the user didn't enter anything for 'blah', how do I create the sql statement so that it will populate the field with 'NA'?

Sadly, when you prepare an insert statement with a particular set of fields using placeholders, the "default" value for a given field in the mysql table definition will never be applied, ever -- even when the associated variable is undef.

To handle that in a way that does not require database details to be embedded in your perl script, you have to use a flexible method of preparing the statement, so that you can leave out fields when their associated values are empty:

sub add_foo
{
    my @expected_fields = qw/bar blah/;
    my ( @insert_flds, @insert_vals );

    for my $f ( @expected_fields ) {
        my $v = shift;
        next unless ( defined( $v ) and length( $v ));
        push @insert_flds, $f;
        push @insert_vals, $v;
    }
    my $sth = $dbh->prepare_cached( 'insert into foo (' .
        join( ',', @insert_flds ) . ') values (' .
        join( ',', ('?') x @insert_flds ) . ')' );

    $sth->execute( @insert_vals );

    return $dbh->last_insert_id( undef, undef, 'foo_id' ));
}
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(that assumes use of RaiseError => 1 in the DBI->connect call to handle error checking).