A slightly simpler map at the cost of declaring another scalar.

$ perl -Mstrict -wle '
> my @count = ( 1, 3, 1, 1, 1, 1, 1, 2, 1, 1, 3, 2 );
> my $idx;
> my @result = map { ( $idx ++ ) x $_ } @count;
> print qq{@count};
> print qq{@result};'
1 3 1 1 1 1 1 2 1 1 3 2
0 1 1 1 2 3 4 5 6 7 7 8 9 10 10 10 11 11
$
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I hope this is of interest.