Isn't blocks*blocksize = (stat $file)[11]*512 the actual used size? 
$ ls -lh sparse_file
-rw-r--r-- 1 aitap aitap 101M Mar 11 20:30 sparse_file
$ du -h sparse_file
1,1M    sparse_file
$ du -B 1 sparse_file 
1060864 sparse_file
$ perl -E'say ((stat shift)[12]*512)' sparse_file
1060864

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Sorry if my advice was wrong.

In reply to Re^4: Usage of a sparse file by aitap
in thread Usage of a sparse file by mishikal

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