"...and modifying the iterator variable modifies the array that you are iterating through ..."

foreach my $a (@ar) ...

You are not using the "traditional" C style for loop. You are using the Perl style foreach loop. From perlsyn:

Big difference. You might be comparing apples to oranges here. Consider this code which actually changes the iterator variable instead of the element.

my @ar = (1, 2, 3);
for my $i (0 .. $#ar) {
  $i = $i + 1;
}

print join ", ", @ar; # prints 1, 2, 3
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UPDATE:
I was attempting to point out that the OP mentioned "modifying the iterator variable" when they are actually modifying the "loop index variable" as perlsyn documents.

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)