open SELF "< $0" or die ...;
flock SELF, LOCK_EX | LOCK_NB or exit;

##</code><code>##

flock DATA, LOCK_EX | LOCK_NB or exit;
...
__DATA__

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;
use Fcntl qw(:flock);

my $highlander;
BEGIN {
    open $highlander, '<', $0 or die "Couldn't open $0: $!\n";
    flock $highlander, LOCK_EX | LOCK_NB or do {
        warn "There can only be one $0\n";
        exit;
    };
}

# Other use statements...

# code...

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;
use Fcntl qw(:flock);

flock DATA, LOCK_EX | LOCK_NB or do {
    warn "There can only be one $0.\n";
    exit;
};

# Code...



# Do not remove double-underscore DATA tag. It is required for highlander process assertion.
__DATA__