OK. Seems i have to use the "Seek thing". This seems to work:

#!/usr/bin/perl
use strict;
use warnings;

open (IN, "+<C:/oracle/ora81/network/ADMIN/SQLNET.ORA") || die "cant o
+pen $!";

my @file = <IN>;

seek IN,0,0;

foreach my $file (@file){
$file =~ s/^SQLNET\.AUTHENTICATION_SERVICES=\ \(NTS\)$/#\ SQLNET\.AUTH
+ENTICATION_SERVICES=\ \(NTS\)/i;
print IN $file;
}
close IN;
[download]

Thanks
MH