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perldoc -f atan2
     atan2 Y,X
...
             relation:
 
                 sub tan { sin($_[0]) / cos($_[0])  }

bash-2.03$ cat ./atan2.pl 
#!/usr/local/bin/perl -w
...
print "angle = $r radians\n";
bash-2.03$ ./atan2.pl 4 3 
angle = 0.643501108793284 radians