atan2

atan2 Y,X

Returns the arctangent of Y/X in the range -PI to PI.

For the tangent operation, you may use the Math::Trig::tan function, or use the familiar relation:

sub tan { sin($_[0]) / cos($_[0])  }

The return value for atan2(0,0) is implementation-defined; consult your atan2(3) manpage for more information.

Portability issues: atan2 in perlport.

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https://perldoc.perl.org/5.20.2/functions/atan2.html