If/when someone posts a Benchmark in this thread, I would predict that a substr() approach will come out ahead. But you might also want to look at Text::Ngram -- maybe instead of an array of 3-grams, you actually want a hash, which has each 3-gram as a hash key, and its frequency of occurrence in the string as the value. From the module's man page: 
  use Text::Ngram qw(ngram_counts add_to_counts);
  my $text   = "abcdefghijklmnop";
  my $hash_r = ngram_counts($text, 3); # Window size = 3
  # $hash_r => { abc => 1, bcd => 1, ... }

[download]
Of course, if you need to keep track of the order in which the 3-grams occur, having them stored as hash keys would not be very useful.

(BTW, you said "I need to take a string and find the number of 3 contiguous letters" -- to be precise, the number of 3-grams in a string is simply  length( $string ) - 2 ;)

In reply to Re: finding number of contiguous letters by graff
in thread finding number of contiguous letters by Marknel

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