Let's call the file with the interesting categories imei_tags.txt and the file with 3 million devices imeis.txt then something like the following would do the job...
WARNING: Untested off the top of my head code

#! /usr/bin/perl

use strict;
use warnings;

open (my $imeitag , '<', '/path/to/imei_tags.txt');
my %devices;
while(<$imeitag>){
  $devices{$1}=0 if /^\s*(\d{8}$/;
}

close $imeitag;

open (my $imei, '<', '/path/to/imeis.txt');
while(<$imei>){
  my $imeitag=$1 if /^\s*(\d{8}\d+\s*$/
  $devices{$imeitag}++ if defined $devices{$imeitag};
}

close $imei;

my $count=0;
for my $imeitag (sort {$devices{$a}<=>$devices{$b}} keys %devices){
  print "$imeitag\n";
  $count++;
  last if $count >=100;
}
[download]

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.