#! perl -slw
use strict;
use threads;
use threads::shared;
use Thread::Queue;

my $sem : shared;

sub worker {
    my $Q = shift;
    while( my $line = $Q->dequeue ) {
         $line =~ s[a][A]g;
         lock $sem;
         print $line;
    }
}

our $T //= 4;
my $n = $T -1;

my @Qs = map Thread::Queue->new, 0 .. $n;

my @threads = map{
    threads->create( \&worker, $Qs[ $_ ] )
} 0 .. $n;

my $i = 0;
while( <> ) {
    $Qs[ $i ]->enqueue( $_ );
    $i = ( $i + 1 ) % $T;
}

$Qs[ $_ ]->enqueue( undef ) for 0 .. $n;

$_->join for @threads;
__END__
junk71 3.5GB >nul