Thanks all of you for your replies. Please find the following code.If you have any further changes to this code please let me know .Also, I don't know how to use text::csv , so if anyone could help, it will be nice.

#!usr/bin/perl
use strict;

open FH, "/home/test1/hello.csv" or die;
my @array,@array1;

#----------I'm pushing all abc's and def's data into respective arrays
+---------------------------

while(<FH>)
{
  if ($_=~/^abc/)
    { 
       push(@array,$_);
     
              }
  elsif($_=~/^def/)
    { 
       push(@array1,$_);
     
              }

           }
#----------I'm writing only one for loop for abc, it will be same for 
+def too. ------------------------------

my @hello;
foreach my $i(0..$#array-1)
{
#-------below code: it splits the second abc and and so on. i.e., stri
+ps second abc from the digits.-------------------

@hello=split  /^\w+\,/,$array[$i+1];


foreach my $j(0..$#hello)
{
#--------below code:this is for replacing \n of first abc,2,3,4,5,6 wi
+th a comma ( ex: abc,2,3,4,5,6, ) 

$array[$i]=~s/\n/,/g;

#--------------below code:here it concatenates the data ( data should 
+look like: 2,3,4,5,6,7,5,2,1,6,2,3,8,2,1,3,1,4 )----------

abc,$array[$i]=$array[$i].$hello[$j];
print $array[$i],"\n";

}
}
# In my above code, the data is concatenated as follows:
abc,2,3,4,5,6 
,7,5,2,1,6,2,3
,8,2,1,3,1,4

#But expected output is :
abc,2,3,4,5,6,7,5,2,1,6,2,3,8,2,1,3,1,4
[download]

Thank you for attempting to post your code -- but this is not the code you are using. Please use a copy and paste function so we are getting the actual code you are using. There's no point in me debugging a typo on PerlMonks that isn't actually in your code.

Here's how I know this is not the code you are using:

C:\Steve\Dev\PerlMonks\P-2013-10-18@1712-DataMergeCSV>mergetest1.pl
Global symbol "@array1" requires explicit package name at
    C:\Steve\Dev\PerlMonks\P-2013-10-18@1712-DataMergeCSV\mergetest1.p
+l line 5.
Global symbol "@array1" requires explicit package name at 
    C:\Steve\Dev\PerlMonks\P-2013-10-18@1712-DataMergeCSV\mergetest1.p
+l line 18.
Bareword "abc" not allowed while "strict subs" in use at 
    C:\Steve\Dev\PerlMonks\P-2013-10-18@1712-DataMergeCSV\mergetest1.p
+l line 41.
Execution of C:\Steve\Dev\PerlMonks\P-2013-10-18@1712-DataMergeCSV\mer
+getest1.pl 
    aborted due to compilation errors.
[download]

If you don't know how to resolve the errors listed above, ask about those. I would be glad to help you learn this stuff on any level. But I emphasize I will help. This means you have to be there, too.  :-)

Also, please post your input file (also using a copy and paste function, please), and put it in between

<readmore>
<code>
{your data here}
</code>
</readmore>

tags for easy downloading.

As noted by Laurent_R, what you posted gives us an idea of what your input data looks like, but it's not quite specific enough given that you ask to help format it exactly like the output you want. Can't get the exact output without the exact input.  :-)

Finally, once we have your core code working, we will examine the possibility of using the Text::CSVmodule or something similar, and we can cover how you make that happen at that time.

Let's crawl first, then walk, then run, then, perhaps, even fly.  :-)

One thing is certain as mentioned by other perlmonks before now that one need know how the OP input file is formatted.
Please see if the below codes gives some light.
Using the OP dataset just as was given and using perl split like thus:

use warnings;
use strict;

my $l = 'abc,2,3,4,5,6 abc,7,5,2,1,6,2,3 abc,8,2,1,3,1,4 ....... def,8
+,9,4,5,6 def,5,6,2,1 ';
my %hash;
    my $k;
    for (split/[,. ]+/ => $l){
      if(/\D+?/){
       $k = $_
      }else{
      push @{$hash{$k}},$_
      }
    }

 print $_,',',join (',' => @{$hash{$_}}),$/ for sort keys %hash;
[download]

Secondly, since your input is a CSV file, it is better you use module like Text::CSV or Text::CSV_XS.
See an example below with a modified OP dataset.

#!/usr/bin/perl
use warnings;
use strict;
use Text::CSV;
use Data::Dumper;

my %hash;
my $csv = Text::CSV->new( { binary => 1 } )
  or die "can't use CSV" . Text::CSV->error_diag();

while ( my $row = $csv->getline(*DATA) ) {
    push @{ $hash{ shift @$row } }, @$row;   # as previously mentioned
+ by Eily
}
$csv->eof or $csv->error_diag();

print Dumper \%hash;

 __DATA__
 abc,2,3,4,5,6 
 abc,7,5,2,1,6,2,3 
 abc,8,2,1,3,1,4
 def,8,9,4,5,6 
 def,5,6,2,1
[download]

If you tell me, I'll forget.
If you show me, I'll remember.
if you involve me, I'll understand.
--- Author unknown to me

# In my above code, the data is concatenated as follows:
abc,2,3,4,5,6 
,7,5,2,1,6,2,3
,8,2,1,3,1,4

#But expected output is :
abc,2,3,4,5,6,7,5,2,1,6,2,3,8,2,1,3,1,4
[download]

Again, we need to see the input file correctly formatted before we can say for sure what you need to do, but it seems very very likely that your problem comes from the fact that the input is split into separate lines, each of which having a trailing end-of-line character. In this case, you simply need to remove that character from your input lines immediately after you read it (although it could be done later, it is best to do it right away). Change your code as follows:

while(<FH>)
chomp;
{
# ...
[download]

There are many other things to be said about your code, but this correction should at least solve your immediate problem. I'll wait for a correctly formatted input before making any further comments.