How do I set an environment variable in Perl?

You can do it like this:

$ENV{HOME} = 'something different';

But please note that this will only have an effect within the rest of your script. When your script exits, the calling shell will not see any changes.

As perldoc -v %ENV says:

%ENV The hash %ENV contains your current environment. Setting a value in "ENV" changes the environment for any child processes you subsequently "fork()" off.