Untested, but it usually works, and written in a way that if this is homework, someone will notice.

sub slurp { local *ARGV; @ARGV = @_; <> }

my %where;

$where{$_} .= "1" for slurp($file1);
$where{$_} .= "2" for slurp($file2);

for (sort keys %where) {
  my $where = $where{$_};
  if ($where =~ /12/) {
    print "both";
  } elsif ($where =~ /1/) {
    print "left";
  } else {
    print "right";
  }
  print ":\t$_";
}
[download]

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update: Yeah, oops, thanks akho. Fixed.