use IO::Socket;
use POSIX qw(WNOHANG setsid);

sub daemonize {
        $SIG{CHLD} = 'IGNORE';        # Configure to autoreap zombies
        die "Can't fork" unless defined ( my $child = fork );  # FORK 
+<<<<<<<<<<<<
        CORE::exit(0) if $child;      # Parent exits
        setsid();                     # Become session leader
        open( STDIN,  "</dev/null" ); # Detach STDIN from shell
        open( STDOUT, ">/dev/null" ); # Detach STDOUT from shell
        open( STDERR, ">&STDOUT"   ); # Detach STDERR from shell
        chdir '/tmp';                 # Change working directory
        umask(0);                     # Reset umask
        $ENV{PATH} = '/bin:/sbin:/usr/sbin';  # Reset PATH
        }
[download]

You can read more in these threads: 201937, 539098.

I am unable to use this function in proper way.. When I am calling this function instead of fork , it just detaches from ssh session but cant process the exact function system(...) that we are really intended for.. I am giving u the modified code... Please come up with exact implementation of daemonize() which will both run the intended function in background..and detatch from ssh session.

#!/usr/bin/perl -w
use strict;
use warnings;
use Getopt::Long;

#variable declaration
my $exec_mode;
my $param;
my $paramfile;
my $log_loc= "/logs/default_log.txt";
my $interv =2;

GetOptions( "e=s" => \$exec_mode, "p=s" => \$param, "pf=s" => \$paramf
+ile, "l=s" => \log_loc, "i=i" => \$interv);

    if($exec_mode eq 'start')
    {
        if( $paramfile eq NULL)
        {
               &startsar ($param, $interv);
        }
        else
        {
               &startsar ($paramfile, $interv);
        }
    elsif($exec_mode eq 'stop')
                            {
            &stopsar ( $log_loc);
                            }
    else
              {
    print " usage sarexec -arguemrnts are-\n -e <start>/<stop>\n -p <p
+arams> \n -pf <param_file> \n -l <location> (by default location is '
+/logs/default_log.txt')\n -i <interval> ( by default interval is 2)\n
+ "
              }

sub startsar 
{ 
              
              if (substr($_[0],0,1) eq "-")
                {
                    my $pid = fork();
                            if($pid>0)
                            {
                             #parent
                            }
                           else
                           {
                            system("sar  -o logfile $_[0]  $interval 0
+ > /dev/null&"); # this utility must run in background and able to cl
+ose the script
        exit 0;
                           }   
                        
                }
                else 
                {
                  chomp $_[0];
                  my $param;
                            open FH,"< $_[0]" or die"cant open the fil
+e";
         $param = <FH>;
         close FH or die "cant close the file";
         chomp($param);
                            my $pid = fork();
                            if($pid>0)
                            {
                             #parent
                            }
                           else
                           {
                            system("sar  -o logfile $param   $interval
+ 0 > /dev/null&"); # this utility must run in background and able to 
+close the script
        exit 0;
                           }
                  }
}

sub stopsar
{
    system("kill \$(pidof sadc)");
      system("sar -f logfile>>$_[0]");
}
[download]

You need to call daemonize before you start running other stuff. You should split your GetOptions subroutine into two. One that just gets the options and does any inilization stuff (e.g. variable assignment) that's needed. Then call daemonize. And then start doing your system calls.