Still executes in a loop, but gets all instances of each letter each time (thus avoids the even/odd issue you have).

use 5.010;
my $str = "abcdefghiaabccdjklm";
my $substr;
$substr = "" while $str =~ s/(.)(?:(.*?)\1(?{ $substr .= $^N }))+/$sub
+str/;
say $str;
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