Can't you just use truncate to create a large file?

$ touch myfile
$ ls -l myfile
-rw-r--r-- 1 john john 0 2007-07-13 12:30 myfile
$ perl -e'truncate "myfile", 200_000_000'
$ ls -l myfile
-rw-r--r-- 1 john john 200000000 2007-07-13 12:31 myfile
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