If I recall correctly, the hash function has changed over time. So it may very well be the case that the hash value of "Somestring" is different in Perl 5.005, 5.6.1 and 5.8. Try the following with different Perl versions:

$ perl -MDevel::Peek -e 'Dump({ key => "value" })'

and look for the line with

Elt "key" HASH =

in it. The hash values are different.

Arjen