Quite simple, perl (and most other programming languages) take angles for trig functions in radians, while calculators take them in degrees, radians or gradians (usually). Most people prefer degrees, but radians are nicer to mathamaticians and other similar people.

The best thing for you to do is

use Math::Trig;

$num = sin( deg2rad( 45 ) );
[download]

I think Math::Trig is part of the standard perl distribution, so read the documentation and go for it.

Update: I know about the math of radians as jepri explained but after the amount of it i've done at school recently I didn't feel like providing an explanation.

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