my $diff = ($new*10000) - ($old*10000);

[download]
Or if you want to round it, 
my $diff = 0 + sprintf '%.0f', ($new*10000) - ($old*10000);

[download]

Update: The *10000 can be factored out: ($new-$old)*10000

In reply to Re^3: Calculating base difference of numbers by ikegami
in thread Calculating base difference of numbers by Anonymous Monk

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