if ($foo[100000] == 1) {
    ...
}

##</code><code>##

[
    undef,
    undef,
    undef,
    undef,
    [ undef, undef, undef, undef , undef ],
]

##</code><code>##

$matrix[4][4] = undef

##</code><code>##

[
    undef,
    undef,
    undef,
    undef,
    [ undef, undef, undef, undef , undef ],
]