this is a very iffy subject i couldn't find clear documentation on with examples. from the documentation i've read on undef(), doing this:
UPDATE on the following code

undef @array; #@array undefined
undef $scalar; #$scalar will be undefined
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will clear those datatypes.. whereas

@array[2] = undef; #pick element number to undefine
$scalar = undef; #$scalar will exist, but with no value
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would assign undefined values to the datatypes. The biggest use for this would be assigning $variables to some array elements.. ie:

($value1,undef,undef,$value2,$value3,undef) = @array;
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(thank you crazyinsomniac)