TIMTOWTDI ... IMHO you should choose the best self documenting approach.

In this case I'd use a variable $index (or $idx ) for arrays and for other loops an auto-incremented $counter++

The loop may run longer due to the increment operation, but

       print ' ' x $indentation if $idx == 0;
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is always readable and can also be extended for last line ( $idx == $#array ) or some modulo intervals

Mr Muskrat already showed you how each can be used to get index and value in one go.

Please be aware that your flag-approach can be better readable in other cases.