my $string = 'abcdefgxxabcdefgzzabcdsjfhkdfab';

my %seen;
1 while $string =~ /(\w{2,}?)(?{$seen{$1}++})(?!)/g;

print [sort {
           $seen{$b}  <=> $seen{$a}
        || length($b) <=> length($a)
    } keys %seen]->[0];