use warnings;
use strict;

my %hash_rec;
$hash_rec{id} = "001";
doSub1(\%hash_rec);

print "back main, id=$hash_rec{'id'}, id2=$hash_rec{'id2'}\n";

sub doSub1 {
    my ($hash_ref) = @_;
    print "inside doSub1, id=$hash_ref->{id}\n";

    # you're not assigning a reference to $hash_ref->{id2},
    # you're assigning a simple string

    $hash_ref->{id2} = "002";

    # you're only sending a scalar string to doSub2()...
    # $hash_ref->{id2} eq "002"

    doSub2($hash_ref->{id2});
}
sub doSub2 {
    # $id2_ref is NOT a reference... it's the value of
    # $hash_ref->{id2}, which is a scalar string only

    my ($id2_ref) = @_;
    print "inside doSub2, id2=$id2_ref\n"; # no need to deref this!
}