sub limit_instance($$);    # proto

use constant MAX_INST => 5;

my $clones = limit_instance( $0, MAX_INST );

print 'There is/are ', $clones, ' of me running.', "\n";
sleep 60;                  # in real life, you would do something here

exit;

sub limit_instance($$) {

    # takes two args:
    # 1: command line name of program to limit ($0 is the current Perl file)
    # 2: maximum number of instances
    use Proc::ProcessTable;
    use integer;
    my ( $program, $limit ) = @_;
    return undef unless ( defined($program) && defined($limit) );
    my $p     = new Proc::ProcessTable;
    my $count = 0;
    foreach my $process ( @{ $p->table } ) {
        $count++ if ( $process->{'cmndline'} =~ m/$program/ );
        die "Limit of $limit instances passed.\n" if ( $count > $limit );
    }
    return $count;
}