When passing parameters, you want to pass scalars - which means you want to pass:

   * scalars as themselves
   * each array as an array reference (a reference is a scalar)
   * each hash as a hash reference (a reference is a scalar)
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so

  1. there is no reason to pass a reference to
     $template(a scalar) to the subroutine - why not just
     pass $template?  Forget about the reference -
     then in the subroutine you can do

        my ($output, $input) = @_;

     and just use $output in the subroutine, instead
     of having to use $$output.

  2. when you pass \@temp_fyle in the call to
     subroutine file_expansion, that's fine.  This is
     just my preference, but IMHO it helps keep your
     code simpler and more understandable if you name
     variables consistent with what they contain.  For
     example, in subroutine file_expansion, name the
     imcoming parameter that is a reference to the array
     what it is - a reference, like this:

         my ($output, $input_arrayref) = @_;

     or maybe even better

         my ($output, $temp_fyle_arrayref) = @_;

     Then, since you can actually use the reference to point
     at array elements (instead of having to dereference
     the array reference into an array), you can do something
     like this

         print "first element is $temp_fyle_arrayref->[0]\n";

     to refer to the 1st element of the array, instead of
     having to do
 
         my @new_array = @$temp_fyle_arrayref;
         my $first = @new_array[0];
         print "first element is $first\n";

     which takes more memory since you are creating another
     array(@new_array).
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HTH.