Take a look at pos.

$ perl -e '
    my $str =   "BaBBaBBBaBBBB";
    # start pos: 0 2  5   9
    #   end pos: 0  3   7    12

    my $fmt = "%d -> %2d\n";

    while ($str =~ /(B+)/g) {
        my $len = length $1;
        my $pos = pos $str;
        printf $fmt, $pos-$len, $pos-1;
    }
'
0 ->  0
2 ->  3
5 ->  7
9 -> 12
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Edit: s/look as pos/look at pos/ Thanks LanX.