open(DATA_IN, ">>$template_data") || print ...
...
   $_ = <DATA_IN>; # No, you can't do this on an output file
   ...

##</code><code>##

my $filename="test.txt";
my $template_data = "unsecure/".$filename;

$/ = undef;
open( DATA_IN, $template_data ) or die "$!\n";
$_ = <DATA_IN>;  # read the whole file into $_;
close DATA_IN;

# now check for patterns in $_ that need to change, 
# and change them.  I'm not clear yet on this part
# of the problem... but when this is done, finish with:

open( DATA_OUT, ">$template_data.new" ) or die "$!\n";
print DATA_OUT;
close DATA_OUT;
__END__