even if the array is presized

If you specify the slice in the assignment it's fine though:

use strict;
use warnings;
use Test::More tests => 1;

$_ = "Anyway I suspect you cannot avoid them repeated: you are are in 
+left side";
my @undef;
my ($key, $val);

($key, @undef[0..4], $val) = split(/\s+/, $_);
is ($val, 'them');
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I still prefer Eily's approach, however.