i use this on scripts that run from cron so that multiple copies don't run at the same time: 
BEGIN {
    use Fcntl qw(LOCK_EX LOCK_NB);
    open SELF, '<', $0 or die "Failed to open $0: $!";
    flock SELF, LOCK_EX | LOCK_NB  or exit;
}
i got a warning from perlcritic about bare filehandles so i converted it to a lexical variable, but then the flock stopped working. not sure why this doesn't work, because the pod for flock shows an example with a lexical variable: 
BEGIN {
    use Fcntl qw(LOCK_EX LOCK_NB);
    open my $self, '<', $0 or die "Failed to open $0: $!";
    flock $self, LOCK_EX | LOCK_NB  or exit;
}

In reply to how to convert lexical filehand so flock will work with it by Anonymous Monk

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