The way to quickly go through and replace NUM with a constant is:

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

But it sounds like you have some rule for wanting to determine what to replace NUM with.

for( @s_dn )
{
  my $num = 1; # calculate the local num, somehow.
  s/NUM/$num/g;
}
[download]