(to the OP) - hash keys are always regarded as strings - that means you should make sure your numeric keys are represented as exactly the right string. I.e. $hash{1.0} is not the same as $hash{1}*. One way of doing that is to use sprintf. For example, if all your keys have 2 places behind the comma, you can use something like this:

my $key = sprintf("%0.2f",$number);
my $value = $hash{$key}
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It actually gets weirder than that -- the . (concatenation) operator has higher precedence than a hash's auto-stringification. This confused a somewhat baffling bug for a few hours recently...

use Data::Dumper;

$x{a}     = "bare a";
$x{"a.b"} = "quoted a.b";
$x{a.b}   = "unquoted a.b";
print Dumper(\%x);'

__DATA__
$VAR1 = {
          'ab' => 'unquoted a.b',
          'a.b' => 'quoted a.b',
          'a' => 'bare a'
        };
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a.b cannot be auto-stringified because it does not match the pattern ^\w+$. What you're observing is the valid perl a.b becomng 'a'.'b' because strict allowed your bareword strings to be unquoted. At this point this has nothing to do with being a hash key. It's just that they're an expression. Enable strict to see what you were missing.

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Thanks to all for your help.
My mistake was effectively to have another hash where the keys were not quoted hence the "undef" for key 5.00 when multiplying. I hope I did not abuse of your time. Many thanks again.