You can confirm that this issue is isolated to the range operator by first taking it out of the for loop and then throwing in a translation with map.

Problem still exists below, so it's not some optimization of the for loop:

my @a = (1..999);
for my $i (@a) {
[download]

Problem goes away due to using integers not created by the range operator:

my $j = 0;
my @a = map {++$j} (1..999);
for my $i (@a) {
[download]