Floating point in perl is fine, but the exponentiation operator in perl is **, not ^.

$temp = 40;
$vol = 2.5;
$pressure=(-16.6999)-(0.0101059*$temp)+(0.00116512*($temp**2))+
   (0.173354*$temp*$vol)+(4.24267*$vol)-(0.0684226*($vol**2));
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