My guess would be that you are trying to get the value like this:

$foo = $hash{5.00};
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which perl is interpreting as:

$foo = $hash{5};
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What you want is this:

$foo = $hash{'5.00'};
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To further illustrate, check what the following code does:

$hash{5} = "foo";
$hash{'5.00'} = "bar";
print $hash{5}, $hash{5.00}, $hash{'5.00'};
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-driver8