my $tree = 'Foo::Bar::Baz';
my %pkg;

my $ref = \%pkg;
for(split /::/,$tree){
   $ref = exists $ref->{$_} ?
        # we don't need no autivivification
       ( $ref->{$_} = {} ) : $ref->{$_}
}

use Data::Dumper;
warn Dumper(\%pkg);