Seems weird. And what really smells suspicious is that if you switch the order of the alternatives, the second string suddenly matches the regex. 
for my $regexp (qr/^((a*)b|a*b?d)*c\2$/, qr/^(a*b?d|(a*)b)*c\2$/) {
    for my $string (qw( aabadcaa aababdcaa )) {
        say $string =~ /$regexp/ ? 'Yes' : 'No';
    }
}

[download]
Output: 
Yes
No
Yes
Yes

[download]

If you want to know what Perl's regex engine does, not just guess, run the script with 

use re 'debug';

[download]

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

In reply to Re: Precise backreference semantics in Perl regular expressions by choroba
in thread Precise backreference semantics in Perl regular expressions by Gro-Tsen

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