You need to use the sub keyword when declaring/defining subroutines. Here's a working version based on your code.

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";
}
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