return the array

You cannot return an array from a C function. Your c code example is returning the modified value of the first element of the array passed (by reference).

That means that in the following C calling code:

...
double rv = 0, ary[ 10 ] = { 0, };

rv = do_nothing( ary, sizeof( ary );
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After the call, rv would contain 1.0, the modified value of ary 0 ; and ary would have been modified to contain 10 x 1.0.

So what do you want the function -- as seen from Perl - to do?

• Take a reference to an existing perl array and modify it in place?
• Take a reference to an array and return a list of values derived from the contents of the passed array and modified by the C function?
• Take a list of values, and return a list of modified values?
• Take a reference to an array and return a reference to a new array containing the modified values?

All of these are possible, but only the first can be written in C alone; all the others would need to be written in XS code.