$x = 4;
@b = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11);

$a[0] = $x;
push @a, @b for 1..$x;
push @a, 0 while $#a < 48;

print "@a";
[download]

This makes very little logical sense, of course, because you say you want a maximum of 49 elements, yet if @b has 11 elements, then any value of $x larger than 4 will go over the 49. It might be more helpful if you gave us a sample of your actual input data and an example of what you want it to come out like.