#! /usr/bin/perl
use strict;
use warnings;
use Fcntl qw( :flock );

print "Script $0 is running now.\n";

my $lock = Check_Race_Condition();

sleep( 100 );
print "End.\n";

sub Check_Race_Condition {
    open( my $self, '<', $0 )
      or die( "Another $0 is already running.\n");
    flock( $self, LOCK_EX | LOCK_NB )
      or die( "Another $0 is already running.\n");
    return $self;
}