Nothing ("()") in scalar context evaluates to undef, so

if (!$myarray[$arrayKey[$i]]) { $myarray[$arrayKey[$i]]=() }
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is really

if (!$myarray[$arrayKey[$i]]) { $myarray[$arrayKey[$i]] = undef; }
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If $myarray[$arrayKey[$i]] is non-existent, 0, the empty string, "0" or one of the few other things that are false, undef will be assigned to it.

Unless you are trying to micro-optimise memory usage (undef takes less memory than the string "0"), it sounds like a rather useless thing to do.

Maybe the author was trying to stretch the size of the array, but that's probably a useless thing to do too. As the statement demonstrates, using and assigning to non-existent elements is not a problem.