use strict;
use warnings;

my ($file1, $file2) = @ARGV;

my $ids = 1000000;  # last id
my $bin=0;
substr($bin,$_,1,pack ("c",0)) for (0..$ids);

# Create the index
open my $fh1, "<", $file1 or die $!;
while (my $id = <$fh1>){
  chomp $id;
  substr($bin, $id,1,pack ("c",1));
}
close $fh1;

# Search $ids from file 2
open my $fh2, "<", $file2 or die $!;
while (my $id2 = <$fh2>){
  chomp $id2;
  print "$id2\n" if ((unpack "c",substr ($bin,$id2,1)) == 1);
}
close $fh2;