This has nothing to do with perl. /proc/$$/environ is only the environment that the process was started in. %ENV does properly export variables, environ simply never changes after a process has started. Observe:

$ perl -e'%ENV=();exec "sh --noprofile"'
$ cat /proc/$$/environ
$ export FOO=sausage
$ cat /proc/$$/environ
$ sh --noprofile
$ cat /proc/$$/environ 
FOO=sausage
$ perl -e'%ENV=(BAR => "sausage");exec "sh --noprofile"'
sh-2.05b$ cat /proc/$$/environ
BAR=sausage
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Perl works as expected, /proc/$$/environ simply isn't what you thought it was.