my $finish   = 'A val that is guaranteed not to be present in any list';
my @list     = ($fh1, $fh2, $fh3, $fh4, $fh5);
my $fetch    = sub {
    my $fh = shift @_;
    return $finish if eof $fh;
    return scalar <$fh>;
};
my $compare  = sub {
    my ($line1, $line2) = @_;
    my ($stamp1) = $line1 =~ /^(\d+)/;
    my ($stamp2) = $line2 =~ /^(\d+)/;
    return $stamp1 <=> $stamp2;
}
my $next = gen_merger(\@list, $fetch, $compare, $finish);
while (1) {
    my $item = $next->();
    last if defined $item && $item eq $finish;
    print "$item\n";
}