my $str = 'aabcdabcabcecdecd';

local our %counts;
$str =~ /
   (.{2,})   # or (.+)
   (?(?{ !$counts{$1} })(?=.*\1))
   (?{ ++$counts{$1} })
   (?!)
/x;

use Data::Dumper;
print Dumper \%counts;