CPAN is your friend!

Using the List::Compare module:

use strict;
use  List::Compare;

my @list_one = qw/user01 user02 user03/;
my %hash_two = qw/user01 user01@emailaddress.com user03 user03@emailad
+dress.com user04 user04@emailaddress.com/;
my @list_two = keys %hash_two;

my $lc = List::Compare->new(\@list_one, \@list_two);

my @results = map {$_ . ' '. $hash_two{$_}} $lc->get_Ronly;

print join "\n", @results;
[download]

Result: user04 user04@emailaddress.com

Update: I quickly made two files of about 190,000 records and used those to see how long it took. Not including the reading in of the files, the script zipped through all 190,000 records in 14 seconds (on a Windows XP, Athlon XP 1600+, Activestate Perl 5.8.7, running within the Komodo-editor).