This is no way addresses your question about optimization, but you could take advantage of Slices to reduce some of your code. For example, replace: 
my $cgram = join " ", $unigrams[$i], $unigrams[$i+1], $unigrams[$i+2],
+$unigrams[$i+3],$unigrams[$i+4];

[download]
with: 
my $cgram = "@unigrams[ $i .. $i + 4]";

[download]
The double quotes around the array slice also insert a single space between array elements ($").

In reply to Re: statistics of a large text by toolic
in thread statistics of a large text by perl_lover_always

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