1a. Because the compiler ignored the mismatched 'end for' and instead put all of the code up to the next 'end foreach' into the inner loop.

1b. Get a compiler with better error handling.

2.

sub sum { return md5sum(join('',@_)); }
my @non_in_place = reverse @array;
my $correct = sum(@non_in_place);
while ($correct ne sum(@array)) {
    ($a,$b)=(rand(@array),rand(@array));
    @array[$a,$b] = @array[$b,$a];
}
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