I'm running Strawberry Perl on Windows 7 and it adds its bin directory to the path automatically. I didn't do anything special and certainly didn't edit the system path manually.
I get:
C:\Users\ian>perldoc -u -f atan2
=over 8
=item atan2 Y,X
X<atan2> X<arctangent> X<tan> X<tangent>
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the C<Math::Trig::tan>
function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
The return value for C<atan2(0,0)> is implementation-defined; consult
your atan2(3) manpage for more information.
=back
C:\Users\ian>
Maybe the installer wasn't run as administrator, so it coulnd't edit the system path, on Windows Vista or later?