use Inline 'C', <<'END_C'

void _munge( char *outlist, int size, char *inlist1, char *inlist2 )
{
    double *output = (double *) outlist;
    double *list1  = (double *) inlist1;
    double *list2  = (double *) inlist2;
    int i;
    for( i= 0; i < size; ++i ) {
        printf("%f %f\n", inlist1[i], inlist2[i]);
    }
}

END_C
    ;

my $size = 3;
my $out = "\0" x ( $size * length pack "d", 0.0 );
my $pack1 = pack "d$size", 1, 2, 3;
my $pack2 = pack "d$size", 4, 5, 6;
_munge( $out, $size, $pack1, $pack2 );

##</code><code>##

0.000000 0.000000
0.000000 0.000000
0.000000 0.000000