@s_dn = map { s/NUM/1/; $_ } @s_dn;
[download]

for( @s_dn )
{
  my $num = 1; # calculate the local num, somehow.
  s/NUM/$num/g;
}
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_
@s_dn=("c=US,ou=NUM,cn=foo", "c=DE,ou=NUM,cn=foo");
@dn=@s_dn;
my $number=0;
   for(my $i=0; $i<3; $i++,$number++)
   {
           for(@dn) { s/NUM/$number/g}
           # now I would save this dn and other stuff now.
           print @dn;
           print "\tou= should be $number\n";
   }
[download]

Your outer for loop is doing nothing of any consequence. Your inner for loop is substituting all occurrences of NUM in @dn, leaving no occurrences left for your subsequent iterations of your outer loop.

Perhaps you need to re-investigate your logic?

The line @dn=@s_dn; should moved to just inside the first for loop, so that the original template is restored each time before you do the substitution. Otherwise, as you probably found, no substitutions will occur after the first iteration.

Thanks that pointed showed 2 things I'm doing wrong. 1) the restoring. when I put that back I had the same issue on my script but not on the snipit given here the difference is I'm using a reference to an array in the script and so I'm not doing a copy I'm doing a ?pointer? is there anyway to stop that?