Script starts.  Am I the only instance of this code?
  --  If YES, continue
  --  If NO, terminate immediately

##</code><code>##

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

my $MARKER_FILE="/home/me/MARKERFILE";

sub checkMarker
{
        if(-e $MARKER_FILE){
                # Marker file already exists, another instance of this script is running!
                return undef;   # FALSE
        }
        else
        {
                # Marker file doesn't exist!  We should create one...
                my $cmd = "touch $MARKER_FILE";
                `$cmd`;
                return 1;       # TRUE
        }
}

package main;

if (checkMarker()) {
        print "I can do stuff!\n";
        # ...do stuff...
        my $endCmd = "rm $MARKER_FILE";
        `$endCmd`;
}
else{
        print "Another instance of this script is running, I can't run...\n";
}