G'day punitpawar,

"Is there a way by making use of s/// ..."

You're starting with a solution and then trying to make it fit a problem. This is back-to-front: start with a problem and then find an appropriate solution.

The output of your second example (cdghijkmnoestsr) contains an 'e' which shouldn't be there: the original string (abcdeffghijkllmnoppqestqaserb) contains three 'e's.

As others have already noted, s/// does not seem appropriate. Here's how I might have tackled this problem:

#!/usr/bin/env perl -l

use strict;
use warnings;

my @test_strings = qw{
    abcdefghiaabccdjklm
    abcdeffghijkllmnoppqestqaserb
}; 

remove_duplicate_chars($_) for @test_strings;

sub remove_duplicate_chars {
    my $str = shift;

    my %seen;
    my @chars = grep { ! $seen{$_}++ } split //, $str;
    my $uniq = join '', grep { $seen{$_} == 1 } @chars;

    print "IN:  $str";
    print "OUT: $uniq";

    return;
}
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Output:

IN:  abcdefghiaabccdjklm
OUT: efghijklm
IN:  abcdeffghijkllmnoppqestqaserb
OUT: cdghijkmnotr
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