#!/usr/bin/perl -w

    use strict;


    die "Usage: $0 subset serial\n" unless @ARGV == 2;

    my $subset = shift;
    my $serial = shift;

    open SUBSET, $subset or die "Couldn't open $subset: $!\n";
    open SERIAL, $serial or die "Couldn't open $serial: $!\n";


    while (<SUBSET>) {

        my $subset_num = $_;
        my $serial_num;

        print $serial_num while ($serial_num = <SERIAL>) < $subset_num;
    }

    print while <SERIAL>;


    __END__