I'm afraid you cannot influence your parent's environment from inside the Perl script, and certainly not by setting %ENV keys. Those variables only get propagated 'downwards' in the process tree, towards children of your perl script, not upwards towards the parent.
As phaylon mentioned,
perldoc -q environment
describes a hack whereby you output the VAR=value commands from your script and pull that through the shell's eval function, but then you're no longer inside your perl script. And it might not work with all shells either.