Is this what you mean?

#!/usr/bin/perl -w
#########################
use strict;

while (my $line=<DATA>) {
  chomp $line;
  unless ( $line =~ /2003/ ) {
    $line =~ s/($line)/$1 2003/;
  }
  print $line,"\n";
}


__END__
Copyright 2001
Copyright 2001-2003
Copyright 2001,2002,2003
Copyright 1999-2001
[download]

When run yields:

--$  perl tack_on.pl
Copyright 2001 2003
Copyright 2001-2003
Copyright 2001,2002,2003
Copyright 1999-2001 2003
[download]