You are looping over an array you are adding to, so you are never reaching the end.

This is somewhat similar to the following loop

my $i=1; my $end= 2;
while ($i++<$end) {
  $end++;
}
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Note that

y @a = (1);
foreach (sort @a) { push @a, 1; }
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stops immediately, because here you are looping over the anonymous array that is not expanding.

Essentially what you are doing is something like this:

my @a = (1);

for ( my $i = 0 ; $i <= $#a ; $i++ ) {
    push( @a, 1 );
}
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So the loop isn't being restarted or re-evaluated, but because you are extending the thing it's iterating over, it never gets to the end.

The insight moment for me in this thread was getting when it is that an anonymous array is created and used (and therefore the loop is unaffected by modifications to the original array during its execution) as opposed to the original array itself being used during the loop.

Note to self: check copy of PBP to see if this is mentioned.

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an anonymous array is created and used

An anonymous list, not an anonymous array. (Well, all lists are anonymous.) No array is created.

Arguments and return values of functions and subs are placed on the stack as a list of scalars.