Why? To make a slice in this case ...

The particular case I had in mind was the
    (undef, my %section) = split /(SECTION \d+)/, $text;
statement dealing with return values from the split built-in function, and I agree that in this kind of case, assignment to undef is reasonable and IMHO preferable.

However, I was trying to make the additional point that this kind of assignment works with any array or list assignment, e.g.,

c:\@Work\Perl\monks>perl -wMstrict -le
"my @ra = qw(zero one two three four five);
 ;;
 my (undef, undef, $x, $y) = @ra;
 print qq{'$x' '$y'};
"
'two' 'three'
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and that in this sort of case it's (usually) better to use some kind of slice, e.g.,
    my ($x, $y) = @ra[ 2, 3 ];