use strict;
use English;
use Parallel::ForkManager;

# Experiment with this value. I suggest you initially try setting
# it to twice the number of CPU threads you have.
my $MAX_PROCESSES = 8;

$pm = new Parallel::ForkManager($MAX_PROCESSES);

open my $src_FH, '<', 'Huge_source_file.txt' or die "Error opening source file $!";
my $line_num = 0;

LINE: while( my $line = <$src_FH> )
{
    next LINE unless 1 == ($INPUT_LINE_NUMBER % 4);
    
    my $worker_pid = $pm->start;
    
    if( $worker_pid )
    {
        # In parent
        next LINE;
    }
    else
    {
        # Call a subroutine to process the line
        process_line($worker_pid);
        $pm->finish; # Terminates the child process
    }
}