This will demonstrate why popping items off of the list you are iterating over is a bad idea:

use strict;
use warnings;

my @list = 'a'..'z';
for my $letter (@list) {
    my $popped = pop @list;
    printf "letter: %s popped: %s list: %s\n",$letter,$popped,join('',
+@list);
}
[download]

Output:

letter: a popped: z list: abcdefghijklmnopqrstuvwxy
letter: b popped: y list: abcdefghijklmnopqrstuvwx
letter: c popped: x list: abcdefghijklmnopqrstuvw
letter: d popped: w list: abcdefghijklmnopqrstuv
letter: e popped: v list: abcdefghijklmnopqrstu
letter: f popped: u list: abcdefghijklmnopqrst
letter: g popped: t list: abcdefghijklmnopqrs
letter: h popped: s list: abcdefghijklmnopqr
letter: i popped: r list: abcdefghijklmnopq
letter: j popped: q list: abcdefghijklmnop
letter: k popped: p list: abcdefghijklmno
letter: l popped: o list: abcdefghijklmn
letter: m popped: n list: abcdefghijklm
[download]

By the time you reach letter 'm' you have removed enough items from the end of the list that you are now at the new end.

Update - added quote from perlsyn

 If any part of LIST is an array, "foreach" will get very
 confused if you add or remove elements within the loop
 body, for example with "splice".   So don't do that