Well, the first thing that I would do is to look in http://search.cpan.org to see if someone has already published a CPAN module that deals specifically with this log-file format.   Most of the time, no matter what we find ourselves doing, “many someone elses” have already done that same thing before, and there’s a very excellent chance that they left a breadcrumbs-trail of CPAN modules behind them.

When dealing with log-files that are formatted like this one, one of my favorite tools are the functions split and join.   For instance, if we split any of the strings in your example, we get a result such as ('', 'a000', '15057', '37', '7439').   Now, we can manipulate the elements individually, then reconstruct the string from the array using join.

The first incarnation of Perl actually came to be, because of the (ever-present) need to do tasks exactly like yours.