join <(sort -b file1) <(sort -b file2)

##</code><code>##

use warnings; use strict;
use Quantum::Superpositions; 
my $s = do { open my $e, "<", "file1" or die 1; any(<$e>); };
{
    open my $m, "<", "file2" or die 2; 
    while(<$m>) { $_=~/(\S+)/ and $1==$s and print; }; 
}
__END__