One way you could to it is use the ^ operator. When applied on strings, it will output 0 wherever the two are equal, and something else wherever they are different. That way searching for a series of identical characters becomes searching for a series of 0.

for my $position (0..(length($reference_str)-length($small_str)))
{
  my $processed_str = $small_str ^ substr($reference_str, $position, l
+ength($small_str));
  next unless $processed_str =~ m<(
                                      \0{9}
                                    | \0{1} . \0{8,}
                                    | \0{2} . \0{7,}
                                    | \0{3} . \0{6,}
                                    | \0{4} . \0{5,}
                                    | \0{5} . \0{4,}
                                    | \0{6} . \0{3,}
                                    | \0{7} . \0{2,} 
                                    | \0{8} . \0{1,}
                                  )>xms;
   print("Match found! ", substr($reference_str, $position+pos($proces
+sed_str),length($1))) and last;
}
[download]

Untested :)