#!/usr/bin/perl

use strict;
use warnings;
use Tie::File;

my %hash  = ();
my $file  = '/path/to/file.txt';
my $count = 700;

tie my @array,'Tie::File',$file or die $!;
while(keys(%hash) < $count-1){
  $hash{$array[int rand(scalar(@array)-1)]} = 1;
}
untie @array;

print scalar(keys(%hash));
print $_,"\n" for(keys(%hash));