I have had dubious results with flowcharts of production programs ... but perltidy is quite a different matter.   A 4,000-line program is “of moderate size,” at best, but it is still big enough to be difficult to understand if it is inconsistently (un?)documented or formatted.   “Tidying up” the source-code by automatic means, so that the block structure etc. is reflected in the indentation of the source listing, can be a big boost in visual comprehension.   Our eyes are very tuned toward the detection of patterns and the exploitation of visual consistency.   When we have that, we can see things “at a glance.”