#!/usr/bin/perl
use strict;
use warnings;
use feature 'say';

sub groupWithEachString {
    my %hash;
    $hash{$_}++ for (split //, shift);
    my $str;
    foreach my $character (sort keys %hash) {
	$str .= $character x $hash{$character};
    }
    return $str;
}

my $str = "aaabbbabc";
say groupWithEachString($str);

__END__

$ perl test.pl
aaaabbbbc