my $str ='helloworldhellohellohihellohiworld';
my %hash;
# grab all the substrings 2+ chars and count occurences in a hash
for my $i ( 0 ..(length($str) -1) ) {
    $hash{ substr($str, $i, $_) }++ for 2.. (length($str) - $i);
}

# sort on occurences and then in alphabetical order
# only select elements that occur >1 times using grep
my @order = sort { $hash{$b} <=> $hash{$a} 
                             ||
                          $a cmp $b
                  } grep { $hash{$_} > 1 } keys %hash;
print "\nPatterns found:\n\n";
print "$hash{$_} occurrences of $_ \n" for @order;

# now grab dictionary file into a hash
# you can save memory using Search::Dict
open DICT, "c:/windows/desktop/dict.txt" or die $!;
while ($word = <DICT>) {
    chomp $word;
    $dict{$word}++;
}

@words = grep { defined $dict{$_} } @order;
print "\nReal words found in dictionary:\n\n";
print "$hash{$_} occurrences of $_ \n" for @words;

# remove substrings of larger words
@words = sort { length $b <=> length $a } @words;
for $i ( 0 .. $#words - 1 ) {
    for $j ( $i + 1 .. $#words ) {
        $hash{$words[$j]} = 0 if $words[$i] =~ m/\Q$words[$j]/ and $hash{$words[$i]} == $hash{$words[$j]};
    }
}

# regenerate sort order grepping out unwanted substrings (set occurences to zero above)
@words = sort { $hash{$b} <=> $hash{$a} 
                             ||
                          $a cmp $b
              } grep { $hash{$_} } @words;
print "\nBest Matches:\n\n";
print "$hash{$_} occurrences of $_ \n" for @words;
__END__
# sample output

Patterns found:

4 occurrences of el 
4 occurrences of ell 
4 occurrences of ello 
4 occurrences of he 
4 occurrences of hel 
4 occurrences of hell 
4 occurrences of hello 
4 occurrences of ll 
4 occurrences of llo 
4 occurrences of lo 
3 occurrences of elloh 
3 occurrences of helloh 
3 occurrences of lloh 
3 occurrences of loh 
3 occurrences of oh 
2 occurrences of ellohi 
2 occurrences of hellohi 
2 occurrences of hi 
2 occurrences of ld 
2 occurrences of llohi 
2 occurrences of lohi 
2 occurrences of ohi 
2 occurrences of or 
2 occurrences of orl 
2 occurrences of orld 
2 occurrences of rl 
2 occurrences of rld 
2 occurrences of wo 
2 occurrences of wor 
2 occurrences of worl 
2 occurrences of world 

Real words found in dictionary:

4 occurrences of el 
4 occurrences of ell 
4 occurrences of he 
4 occurrences of hell 
4 occurrences of hello 
4 occurrences of lo 
3 occurrences of oh 
2 occurrences of hi 
2 occurrences of or 
2 occurrences of wo 
2 occurrences of world 

Best Matches:

4 occurrences of hello 
3 occurrences of oh 
2 occurrences of hi 
2 occurrences of world