Here is the way I would do it, with IPC::Open3. If you add IO::Select into the mix, you can listen to multiple commands simultaneously ( shown in the second code block). Additionally, if you want a little graphic realtime, see linux memory leak monitor, where I do essentially what is shown below, but output it to a little Tk window in the bottom right corner of the screen. 
#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;

my $pid = $$;

my $pid1 = open3(0, \*READ,0,"top -d 2 -b -p $pid ");
            #if \*ERROR is false, STDERR is sent to STDOUT 


while(1){
  my @words = split(/\s+/,<READ>);
  $words[0] ||= 0;
  if($words[0] =~ /$pid/){print "$words[4]  $words[9]\n"}
#print "$words[0]\n";
}

waitpid($pid, 1);
# It is important to waitpid on your child process,  
# otherwise zombies could be created.

[download]
And for multiple command reading, throw them into a select loop. 
#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;
use IO::Select;

my $pid = $$;

my $pid1 = open3(0, \*READ,\*ERROR,"top -d 1 -b -p $pid ");
#if \*ERROR is false, STDERR is sent to STDOUT

# add more commands here if you want, but make sure the
# filehandles are different. It would be an improvement to use
# IO::Handle for the filehandles


my $sel = new IO::Select();

$sel->add(\*READ);
$sel->add(\*ERROR);

my($error,$answer)=('','');

while(1){

foreach my $h ($sel->can_read){
    my $buf = '';
        if ($h eq \*ERROR){
              sysread(ERROR,$buf,4096);
              if($buf){print "ERROR-> $buf\n"}
}else{

 sysread(READ,$buf,4096);
    if($buf){print "$buf\n"}
   }
  }
}

[download]
I'm not really a human, but I play one on earth.
Old Perl Programmer Haiku ................... flash japh

In reply to Re: Running 'top' command (of linux) for few minutes and then come out of system command (of perl) by zentara
in thread Running 'top' command (of linux) for few minutes and then come out of system command (of perl) by Striker

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