Here is another example, all modules are standard. Also see Obtaining data from the top command via a pipe

#!/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); # a small delay
    my $size = <READ>;
    print "$size";
}
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