first, note that %hash{'key1'}{'key2'} should be $hash{'key1'}{'key2'}

If you're not accessing the hash through it's key order of key1, key2, then you should probably re-hash it. not sure what you're looking for, but this will make a hash keyed on 'string' and has an arrayref of the key2's. (no clue if your string:key2 is 1:1 or 1:many) 
my %strToKey2Hash;
foreach my $href ( values %hash ){
  while( my ($key2,$string) = each %$href ){
    push @{$strToKey2Hash{$string}}, $key2;
  }
}

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In reply to Re: fast hash search by davidrw
in thread fast hash search by ddoc

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