use strict;
use warnings;

my $template  = "index.txt";

open (FILE,"$template") or die "Can't read from $template: $!";
my @file=<FILE>;
close (FILE);

open (FILE,">$template") or die "Can't write to $template: $!";
  foreach (@file) {
    if (/<% INCLUDE file(1|2)\.txt %>/) {
      $1 == 1 ? s/<% INCLUDE file1\.txt %>/<% INCLUDE file2\.txt %>/ :
    s/<% INCLUDE file2\.txt %>/<% INCLUDE file1\.txt %>/;
  }
  print FILE;
}
close (FILE);