I need the first option.

Then do_nothing1() below will get you started:

#! perl -slw
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => '_1159002', CLEAN_AFTER_BUILD => 0
+;

#define IS_VARS Inline_Stack_Vars
#define IS_RESET Inline_Stack_Reset
#define IS_ITEMS Inline_Stack_Items
#define IS_ITEM( n ) Inline_Stack_Item( n )
#define IS_PUSHS( s )   Inline_Stack_Push( sv_2mortal( s ) )
#define IS_PUSHIV( iv ) Inline_Stack_Push( sv_2mortal( newSViv( iv ) )
+ )
#define IS_PUSHUV( uv ) Inline_Stack_Push( sv_2mortal( newSVuv( uv ) )
+ )
#define IS_PUSHNV( nv ) Inline_Stack_Push( sv_2mortal( newSVnv( nv ) )
+ )
#define IS_DONE Inline_Stack_Done

double do_nothing1( AV *x, int sizeOfX ) {
    int index;

    for( index=0; index<sizeOfX; index++ ) {
        av_store( x, index, newSVnv( 1.0 ) );
    }
    return SvNV( *av_fetch( x, 0, NULL ) );
}

void do_nothing2( AV *a, SV *size ) {
    IS_VARS;
    int index;

    IS_RESET;
    for( index = 0; index < SvIV( size ); ++index ) {
        IS_PUSHNV( 1.0 );
    }
    IS_DONE;
}

void do_nothing3( SV*dummy, ... ) {
    IS_VARS;
    int index;

    for( index = 0; index < IS_ITEMS; ++index ) {
        IS_ITEM( index ) = newSVnv( 1.0 );
    }
    return;
}

AV *do_nothing4( AV *x ) {
    IS_VARS;
    AV *r = newAV();
    int index;

    for( index = 0; index < IS_ITEMS; ++index ) {
        av_push( r, newSVnv( SvNV( *av_fetch( x, index, NULL ) ) + 1.0
+ ) );
    }
    return r;
}

END_C

my @a = ( 0 ) x 10;

print do_nothing1( \@a, 10 );
print "@a";

my @b = ( 0 ) x 10;

print join ' ', do_nothing2( \@b, 10 );

print join ' ', do_nothing3( 1 .. 10 );

my @c = ( 1 .. 10 );

print join ' ', do_nothing4( \@c );
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If you have Inline::C installed and you run the above code you'll get this output:

C:\test>1159002.pl
1
1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1
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If you then look in the _inline directory tree created below the cwd where you run this, and look for the file _Inline/build/_1159002/_1159002.xs you'll see how it all fits together: