Just to situate the historical context, I added the if defined $cond much later on in the piece, when I added the IGNORE() function to change an AND() or an OR() into a no-op.

The idea was to ensure that "valid1", "valid2", undef, "valid3" produces a syntactically correct query, without a fourth half-baked conditional creeping in there.

But you know what? You're absolutely correct. Given the above and the following:

sub _joiner {
    my $op = shift;
    my $filter = shift;
    while (my $cond = shift) {
        $filter = "($op $filter $cond)";
    }
    return $filter;
}

my $filter = AND(
    "(a=1)",
    IGNORE(
        "(b=1)",
        "(b=2)",
    ),
    "(c=1)",
);
[download]

It does indeed produce (& (a=1) (c=1)). Thank-you very much for this insight, I appreciate it.