#!/usr/bin/perl -w
use strict;
=head - Per OP
$a='aabbccdddd';
I want the Output like this
$a='abcd';
=cut

my $a = 'aabbccdddd';
my $prev = '';
my @a = split //,$a;
for my $found(@a) {
    if ($prev eq $found) {
        next;
    } else {
        print "\$found is: $found \n"; #Omit \n if oneline output desired.
        $prev = $found;
        next;
    }
}



##</code><code>##

ww@GIG:~/pl_test$ perl -c 718695.pl
718695.pl syntax OK
ww@GIG:~/pl_test$ perl  718695.pl
$found is: a
$found is: b
$found is: c
$found is: d


##</code><code>##

ww@GIG:~/pl_test$ perl  718695.pl
    $found is: a
    $found is: b
    $found is: c
    $found is: a  # dup
    $found is: d