I'm not sure I'm understanding correctly what you want to do but this gives the answer you seem to want.

$ perl -Mstrict -Mwarnings -E '
> my $last;
> my $pos;
> my $stream = q{0123??6789??????};
> map { $last = $pos unless m{\?\?}; $pos ++ }
>    unpack q{(a2)*}, $stream;
> say $last;'
4
$
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I hope this is helpful but perhaps you could clarify your requirement if I have misunderstood.