{
  my @matches;
  my $push = qr/(?{ push @matches, $1 })/;

  sub match_all_ways {
      my ($string, $regex) = @_;
      @matches = ();
      $string =~ m/($regex)$push(?!)/;
      return @matches;
  }
}

sub common_substr {
    my ($str1, $str2, $len) = @_;

    my %substr = map { $_ => 1 } match_all_ways($str1 => qr/.{$len}/);
    $substr{$_} |= 2 for match_all_ways($str2 => qr/.{$len}/);

    return grep { $substr{$_} == 3 } keys %substr;

}

print "$_\n" for common_substr("ABCDEF", "ABDEFCBDEAB", 2);
__END__
DE
EF
AB

##</code><code>##

{
  my @matches;
  my $push = qr/(?{ push @matches, $1 })/;

  sub match_all_ways {
      my ($string, $regex) = @_;
      @matches = ();
      $string =~ m/$regex$push(?!)/;
      return @matches;
  }
}

sub common_substr {
    my ($str1, $str2, $len) = @_;
    my %subs;
    @subs{  match_all_ways("$str1\0$str2" => qr/(.{$len}).*\0.*\1/)  } = ();
    return keys %subs;
}