I agree, the logic can be usefully simplified here. But your suggested change is not correct. With the negations unwound, the equivalent logic is:

if (defined $name and defined $sysoid)
{
    $lookup{$name}{name}   = $name;
    $lookup{$name}{sysoid} = $sysoid;
}
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(But I would normally use the higher-precedence && in this situation in preference to the lower-precedence and, which I tend to reserve for control flow.)

Hope that helps,