$a =~ s/(.)\1+/$1/g;
[download]

could you please explain what is the meaning for \1+ in perl

could you please explain what is the meaning for \1+ in perl

In regular expressions, backslashed numeric digits are references back to the preceding captures. So \1 refers to the same thing that was captured in the first set of parentheses, and \2 refers to the same thing that was captured in the second set, and so forth.

As for the +, it modifies a backreference in exactly the same way it modifies any other atom in a regular expression.

-- 
We're working on a six-year set of freely redistributable Vacation Bible School materials.

$ perl -le'
$_ = "aabbccdddd";
print;
tr///cs;
print;
'
aabbccdddd
abcd
[download]

$ perl -wle '%h = map { print unless defined($h{$_}); $_ => $h{$_}++ }
+ split(//,q{abcdabcdabcd}); '
a
b
c
d
[download]

TIMTOWTDI:

#!/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 desi
+red.
        $prev = $found;
        next;
    }
}
[download]

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
[download]

Note: This will fail on the likes of $a='aabbccadddd'

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

If the data is in this form, use a %hash as andreas1234567 does, above or sort the string.