#!/bin/perl
use v5.12;
use warnings;

       my $in = $ARGV[0];
       if (! defined $in) {
          die "Usage: $0 filename";
       }


       my $out = $in;
       $out =~ s/(\.\w+)?$/out/;

       if (! open my $in_fh, '<', $in ) {
          die "Can't open '$in': $!";
          }

       if (! open my  $out_fh, '>', $out) {
          die "Can't write '$out': $!"; 


##</code><code>##

Usage: ./exer_9_2 filename at ./exer_9_2 line 7.