in.txt


__DATA__

13454
34456
1342
5667
987
3354
145

##</code><code>##

use strict;
use warnings;

my @array;  # contains numbers (1..1000000) ~ 400000 numbers

my @array1 = sort @array; # sort it to speed up the process??

open (in , "<", "in.txt") || die "$!";
while(<in>){
m/^(\d+)/;
my @a = grep {$_ == $1} @array1;
print "@a, " if (@a);
} 
close in;