$#array is the index of the last element. pop shortens the array. In your condition, you have

$index < $#array + 1
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while in your body, you both increase $index and decrease $#array. So, after printing 5, $index is no longer less than $#array + 1, and hence the loop terminates.

Why not

my @array = (1 .. 9);
while (@array) {
    my $last = pop @array;
    print "$last\n";
}
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Or some alternatives:

my @array = reverse (1 .. 9);
print "$_\n" for @array;

my @array = (1 .. 9);
my $index = $#array;
while ($index >= 0) {
    print $array[$index], "\n";
    $index--;
}

my @array = (1 .. 9);
for (my $index = $#array; $index >=0; $index--) {
    print $array[$index], "\n";
}
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