use warnings;
use strict;

my $limit    = 40_000_000;
my $how_many = $limit/2;
my $hits = 0;

open (my $handle, 'rand_sorted.txt') or die "Unable to open rand_sorted.txt: $!";

# Using a C style loop to avoid a large temp list (Grandfather)

for (my $i = 0; $i < $limit && $hits < $how_many; $i++) 
{  
    my $record = <$handle>;
    
    if (rand(2) > 1 || ($limit-$i) <= $hits ) {
        $hits++;
        
        # Do what you must to the record
        print "$i: $record\n";
    }
}

close ($handle);