most probably is foreach internally holding an index to iterate the array.

So it must terminate when the 6th element ( $array[5] ) is requested, because the array is already smaller then.

look at this as demonstration

use strict;
use warnings;

my @array= 1..10;

my $idx=0;
for my $elem (@array) {
   print "<\$array[$idx] == $elem> @array\n";
   shift @array;
   $idx++
}
[download]

out:

<$array[0] == 1> 1 2 3 4 5 6 7 8 9 10
<$array[1] == 3> 2 3 4 5 6 7 8 9 10
<$array[2] == 5> 3 4 5 6 7 8 9 10
<$array[3] == 7> 4 5 6 7 8 9 10
<$array[4] == 9> 5 6 7 8 9 10
[download]

This should be clearer, HTH!