If you mean @x = (1,2,3,4,5,6) then you have what you want.

@x = (1,2,3,4,5,6);
print STDOUT join(" ", map { sprintf "%7d", $_ } @x), "\n";

#outputs:
      1       2       3       4       5      6
[download]

Note, in this and further examples I am using STDOUT as a stand-in for your FILE filehandle. This is simply to make testing easier.

If you really did mean @x = (123456); then you will have to split the elements of @x in a seperate step.

@x = (123456);
print STDOUT join(" ", map { sprintf "%7d", $_ } map { split //, $_ } 
+@x), "\n";

#outputs:
      1       2       3       4       5       6
[download]

Perhaps your confusion lines in the density of the code. Here is the original expanded a bit:

my @x = (1,2,3,4,5,6);
my @new;
            
foreach my $x (@x) {
    push(@new, sprintf("%7d", $x));
}
     
print STDOUT join(" ", @new), "\n";

#outputs:
      1       2       3       4       5       6
[download]

I'll leave it to you to expand the @x = (123456) code.