Ouch! That looks like a bug to me... (unless I'm misunderstanding some deep voodoo) BTW 5.00503 and 5.6.1 behave differently on linux (though neither behave as I would expect...):

% perl5.00503 tamper.pl
My arguments were: 

% perl5.6.1 tamper.pl
My arguments were: All your arguments are belong to us.

% perl5.6.0 tamper.pl 
My arguments were: All your arguments are belong to us.
[download]

Update: Added 5.6.0 test from a different linux machine...
Update2: You might try playing around with local *_ which doesn't do the right thing, but at least it does something different....

-Blake