#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;

my $pid = shift || $$;
my $pid1 = open3(\*WRITE,\*READ,0,"/bin/sh");
 #if \*ERROR is false, STDERR is sent to STDOUT  

while(1){
 print WRITE "ps -o rss= -p $pid\n";
   select(undef,undef,undef,.1);
    my $size = <READ>;
   
 #do your checks here and do whatever
    print "$size";
}