<off-the-cuff answer>

This sounds like it's not a problem with sprintf in either the Perl or C incarnations. It's probably more of a problem with how floats are represented in binary. 0.5, for example, may actually be stored as 0.499999999993, or something like that. That rounds to 0. 1.5 may actually be stored as 1.50000004, which rounds to 2.

</off-the-cuff answer>

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