use strict;
use warnings;

my (%hash);
$hash{'id1'} = "1";
doSub1 (\%hash);

# the return id2 should be changed within doSub2 when doSub1 call it.
print "$hash{'id2'}\n";

sub doSub1 {

# accept param as hash by reference
	my ($hrec_ref) = @_;
	$$hrec_ref{'id2'} = "0";

# doSub2(\$hrec_ref{'id2'}); Return Error. Not valid. (works for me - Hippo)
	doSub2 (\$hrec_ref->{'id2'});

}

sub doSub2 {

# handle only a single string param by reference
	my ($href) = @_;
	$$href = "value changed in doSub2";
}