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.
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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"}
   }
  }
}
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