use Env;
By default, ties global scalars for all variables in shell environment
Can tie to global arrays too:
via an import list, specifying arrays and scalars:
use Env qw[@PATH $USER]; # only tie these variables
push @PATH, '.'; # live update to $PATH
print "$USER's path is:\n ", join '\n', @PATH;
Downside: modifying array variable requires it to re-split the string.