double do_nothing(double x[], int sizeOfX)

My advice is to install Inline::C and try this script:

use strict;
use warnings;
use Math::BigInt;

use Inline C => Config =>
 BUILD_NOISY => 1,
 USING => 'ParseRegExp',
 CLEAN_AFTER_BUILD => 0,
 ;

use Inline C => <<'EOC';


double do_nothing(SV * x, ...)
{
  dXSARGS; /* enable stack manipulation */
  int index;
  for(index=0; index<items; index++)
  {
    printf("%f ", SvNV(ST(index)));
    ST(index)=newSVnv(1.34);
  }
  printf("\n");
  return SvNV(ST(0));
}

EOC

my @in = (2.3, 3.4, 4.5, 6.8);
my $d = do_nothing(@in);
print $d, "\n";

my @next = (-1.7, -1.3, @in);
my $d2 = do_nothing(@next);
print $d2, "\n";

__END__
Outputs:
2.300000 3.400000 4.500000 6.800000
1.34
-1.700000 -1.300000 2.300000 3.400000 4.500000 6.800000
1.34
[download]

Because that script specifies CLEAN_AFTER_BUILD=>0 you can then cd into the ./_Inline/build directory and view the XS file that was autogenerated and used.
(I've never really learned how to write XS - I just use the XS file that Inline::C creates.)

There are of course, other ways to formulate do_nothing() - have a look at perldoc Inline::C-Cookbook for examples of the above and other perl API usage.
And see perldoc perlapi for documentation on the API functions.

In XS, it is allowable to pass a list of "double" types as arguments, but I think passing a variable-sized list of doubles (as you intended) would turn out to be messier than the above demo.
And that's why I've opted to pass the values as a list of "SV*" types.

You could also write do_nothing() as:

double do_nothing(SV * x) {
  ... do stuff
}
[download]

In that case you'd be calling do_nothing() from perl as something like:

my $double = do_nothing(\@in);
[download]

I can give you a demo of that, too, if you like.

As a means to understanding the perl API, I really can't recommend Inline::C strongly enough.
It gives one the opportunity to quickly and easily trial constructs about which one is unsure - and is, IMO, the best available XS/API learning tool.

Cheers,
Rob