If you use a disk based hash, for example a DBM::Deep hash, you won't run out of memory. The main reason to have a disk-based hash is that you can create hashes that are bigger than your memory (another reason to use it is that the hash is permanent).

For sample code just look at the perldoc page of DBM::Deep, under 'Synopsis'. Basically you just call DBM::Deep to link a hashname with a file on disk and after that you use that hash like any other hash, only that everything you store in there is (behind the scenes) transfered directly to disk.

I tried to use it! why when I use in this way, the results are not correct.

sub to_hash {
my $file = shift;
my $db = DBM::Deep->new( "$file.db" );
open(FILE, "<$file");
foreach $l (<FILE>) {
    my ($ngram,$line) = split /\t/, $l;
    push(@{ $db->{$ngram} }, $line);
    }
close FILE;
return $db;
}
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for example when I search for a key, I'll get the correct value few times instead of for example one or two times!