|Don't ask to ask, just ask|
Remember you can use the special variable $^F to control this. If set to 0, uses Celsius; if set to 1, uses Fahrenheit; by default it's set to 2 which auto-detects based on your locale. If you don't believe me (and I have no idea why you wouldn't) then run the following on a sufficiently modern perl:perl -E'say $^F'
I hear that 5.16 is going to modularise temperature scales so that you can download custom ones from CPAN, and come bundled with an additional scale for Kelvin.
Fahrenheit is a funny scale. The Celsius scale is, as everybody knows, based on the freezing and boiling points of water. The freezing point was designated 0; the boiling point 100; and everything in between was divided equally into a hundred degrees. (Although named after Anders Celsius, his scale actually labelled the boiling point 0, and the freezing point 100. The modern Celsius scale was developed after his death and named in his honour.)
Initially I thought Daniel Gabriel Fahrenheit must have followed a similar process but with a different liquid. After all, its zero doesn't seem to bear any relation to water.
But actually it turns out he used water after all. However, rather than dividing it into a scale of 100 parts, he used 180, and rather than defining the freezing point of water to be zero, he defined it as 32, because he knew he could achieve a lower zero by mixing water, salt and ammonium.
This is why, unlike miles versus kilometres, or pounds versus kilograms, you can use a simple formula to convert precisely between Celsius and Fahrenheit.$f = 1.8 * $c + 32;
And it gives you an exact conversion (within the bounds of floating point arithmetic).
For fun, see what happens when you plug $c = -40 into that.