I agree with browser that it's not clear what you want to do.

If you wan't to kill monitor.pl (if it's running) from a separate script, this should work:

#! /usr/bin/perl
use strict;
use warnings;

my $list = `ps -a`;
print STDERR "$list";
my @processLine;
my @processesArray = split("\n", $list);
print STDERR "$processesArray[0]\n";

for (@processesArray)
    {
        if ($_ =~ m/monitor.pl/)
            {
                @processLine = split(" ", $_);
                system ("kill $processLine[0]");
                print STDERR "$processLine[$#processLine] killed.\n";
            }
    }
[download]

On *nix systems at least, because the OS processes the command line before running the original script (ie. "monitor.pl > /dev/null"), I don't know of a way to distinguish instances of a script based on their redirection targets.

I hope that helps.