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

FILE: for my $file ( @ARGV ) {
  # check if $file contains a file name
  if ( !-f $file ) {
     warn "Sorry, '$file' doesn't look like a file!\n";
     next FILE;
  }

  # if we can't open the file for reading, warn user
  if ( !open(my $infh, '<', $file) ) {
    warn "Sorry, couldn't open $file: $!\n";
  } 
  # we could open the file for reading
  else {

    # so, let's try to open a temporary file for writing
    if ( !open( my $outfh, '>', $file.'.new' ) ) {
      # and warn user if it failed
      warn "Sorry, couldn't open $file.new for writing: $!\n";
    }
    else {
      # we now have two filehandles:
      # one to read from and one to write to.

      # read linewise from input filehandle
      while ( my $line = <$infh> ) {

        # print (to output filehandle) original line and append a newline
        print $outfh $line, "\n" or die "writing to $file.new failed: $!\n";
      }

      # writing should be done now; close filehandle
      close $outfh or die "closing $file.new failed: $!\n";
    }

    # reading should be done now; close filehandle
    close $infh or die "closing $file failed: $!\n";
  }
}