Assuming the lines are in order, as shown above...

open(INPA, $inpa) || die "Can't open $inpa";
open(INPB, $inpb) || die "Can't open $inpb";

my $a = <INPA>; chomp($a);
my $b = <INPB>;
while ($a && $b) {
    $b =~ /^(\d+) /;
    if ($a < $1) {
        $a = <INPA>; chomp($a);        
    }
    elsif ($a == $1) {
        print $b;
        $a = <INPA>; chomp($a);
        $b = <INPB>;
    }
    else {
        $b = <INPB>;
    }
}

close(INPA);
close(INPB);
[download]

The advantage of this code is it's simple and easy to edit for other formats by changing the regular expression (currently set for one or more digits followed by a space) and comparisons (change to lt, eq, gt for string keys). It also doesn't require huge arrays or hashes.