#! perl -sw
use vars qw/$N/;
use strict;
no strict 'refs';
$|++;

my $reclen = 37; #! Adjust to suit your records/line ends.
$N = $N || 1;

warn "Usage: $0 [-N=n] file\n" and exit(-1) unless @ARGV;

warn "Reading input file $ARGV[0] ", -s $ARGV[0],  "\n";

if ( not defined $ARGV[1] ) {
	warn "Output file not specified a Continue[N|y]?";
	exit -1 if <STDIN> !~ /^Y/i;
}

$/= \$reclen;

open INPUT, '<', $ARGV[0] or die $!, $ARGV[0];
binmode(INPUT);

my (@fhs);
while ( <INPUT> ) {
	my $key = substr($_, 0, $::N);
	if (not defined $fhs[$key]) {
		$fhs[$key] = "temp.$key";
		warn( "\rCreating file: $fhs[$key] ");
		open( $fhs[$key], ">$fhs[$key]")
			or die( "Could create $fhs[$key]: $!");
		binmode($fhs[$key]);
	}
	print {$fhs[$key]} $_;
}
#! Get rid of unused filehandles or those that reference zero length files.
@fhs = grep{ $_ and ! -z $_} @fhs;

close $_ for @fhs;
close INPUT;

warn "Split made to: ", scalar @fhs, " files\n";

#! Sort the split files on the first & second field
for my $fh (@fhs) {
	warn "$fh: reading;...";
	open $fh, "<$fh" or die $!;
	binmode($fh);
	my @recs = <$fh>;
	close $fh;

	warn " sorting: ", scalar @recs, " recs;...";
	@recs = sort{ substr($b, 0, 5) <=> substr($a, 0, 5)
			||    substr($b, 6, 11) <=> substr($a, 6, 11) } @recs;

	warn " writing;...";
	open $fh, ">$fh" or die $!;
	binmode($fh);
	print $fh @recs;
	close $fh;

	warn "done;\n";
}

warn "Merging files: ";
*SORTED = *STDOUT;
open SORTED, '>', $ARGV[1] and binmode(SORTED) or die $! if $ARGV[1];
for my $fh (reverse @fhs) {
	warn " $fh;";
	open $fh, "<$fh" and binmode($fh) or die $!;
	print SORTED <$fh>;
	close $fh;
}
warn "\nClosing sorted file: sorted\n";
close SORTED;
warn "Deleting temp files\n";
unlink $_ or warn "Couldn't unlink $_\n" for @fhs;
warn "Done.\n";

exit (0);