perl: ambiguous environment

Stephane Chazelas discovered a bug in the environment handling in Perl. Perl provides a Perl-space hash variable, %ENV, in which environment variables can be looked up. If a variable appears twice in envp, only the last value would appear in %ENV, but getenv would return the first. Perl's taint security mechanism would be applied to the value in %ENV, but not to the other rest of the environment. This could result in an ambiguous environment causing environment variables to be propagated to subprocesses, despite the protections supposedly offered by taint checking.