If instead of putting the urls in a hash, you use their md5 digests Digest::MD5 (in binary) as keys, then you will save a subtantial amount of space. 1 million binary MD5s stored as hash keys uses about 36 MB: 

use Digest::MD5 qw[ md5 ];;

undef $h{ md5( $_ ) } for 1 .. 1e6;;

print total_size \%h;;
35665714

[download]

Note also the use of undef $hash{ ... }. This autovivifies the key without allocating any storage for a value--thereby saving space. Whilst some will view this as an unconscionable "trick", for this type of lookup application that is pushing your memory limits, the savings are worth having.

Using this method you should be able to index close to 60 million urls on a typical 2GB machine without problems. And far more quickly than any tie or DB mechanism that requires disk accesses.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
RIP an inspiration; A true Folk's Guy

In reply to Re: Another "out of memory!" problem by BrowserUk
in thread Another "out of memory!" problem by slugger415

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