Yes, it could also be that $amt is intentionally something like 4887.14999..., so my statement above may be a little too certain (edited). That in Perl, rounding is often done with sprintf's %f instead of %d is a good point.

Update: Wait a minute - OP says they did $amt = sprintf("%.2f",$amt); before the $amt = $amt*100;, so the issue is pretty certainly that $amt was first something that rounds to 4887.15, which can't be represented accurately, and assuming $amt is the same as $amount in the root node, the cause of the issue is pretty clear.