How does backing up several bytes work with unpack? (a bug?) (was: Re^2: Can unpack add zero bytes before converting?)

[vr]

I had to explicitly include a count in template: "(QX$pad_len)$count", instead of simply "(QX$pad_len)*", because otherwise @values array would result in a very puzzling length of 2501 items instead of 1000 i.e. $count.

I suspect there's special code somewhere to prevent ((un?)documented(?) case of) endless loops with '(CX)*' or '(vX2)*' or similar, but still:

say for unpack '(VX2)*', "\1\1\1\1";    # why 2 items?
say for unpack '(VX3)*', "\1\1\1\1";

'X' outside of string in unpack         # how's that?

and why 2501 items with '(QX3)*' template to begin with?