my $L = ... number of lines ....;
my $N = ... number of processes ....;

# Open filehandles for each process:
my @fh;
for (my $i = 0; $i < $N; $i ++) {
    open $fh[$i], "| $process" or die;
}
    
# Initialize @A:
my @A = (0) x $N;
     
my $l = 0;
    
# Iterate over the input.
while (<$input>) {
    # Pick a number.  
    my $r = rand($L - $l);

    # Find process.
    my $i = 0;
    while ($r >= ($L / $N - $A[$i])) {
        $r -= $L / $N - $A[$i];
        $i ++;
    }  

    # Write line.
    print $fh[$i] $_;
    
    # Increment array.
    $A[$i]++;
     
    # Increment line counter;
    $l++
}