You said:

At least when the padding is whitespace (which the OP seemed to want)

But OP said:

Is it possible to modify the default pad character of the format function from space to something user defined?

Yes, had the original question been about padding whitespace, I certainly would have preferred sprintf. But it was precisely not about that, hence my solution.

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.