What does this code do?

Question

This does what I would like it to

if (grep {/$dn/} @ad_sys) {
    $is_system = 1;
}

but this always returns 1.

if (grep $_ == $dn, @ad_sys) {
    $is_system = 1;
}

What does the second piece do?

Answer 1

== is used for numeric comparison, if you need string comparison use eq.

Answer 2

It filters those elements from the list @ad_sys that are numerically equal to $dn. Then, if the result is not empty, the condition is true and the if-block is entered.