#!/usr/bin/perl -w

# Mergesort

use IO::Handle;  # For the ->getline
require 5.6.0;   # Sort sub prototypes

$recs=13;   # Total number of records to sort.....
            # Leave out of the real thing
$max=5;     # Maximum number of records per merge file
@files=();

# The prototype is needed because we want lexical
#    values in the sort because we're using it as a
#    regular comparison and as a sort sub.
sub sortsub ($$) { my($c,$d)=@_; return $c<=>$d;  }
{
        # Should be POSIX::tmpnam.  But I'm lazy at the moment.
        #   (Under UNIX you can even re-use the same name each
        #   time and just unlink it after the push()!)
        $tempname="fooaa";
        sub store {
                my($a)=@_;
                my $f;
                open($f, "+>/tmp/$tempname") || die;
                print $f sort sortsub @$a;  # Sort small pile
                seek $f, 0, 0 or warn "Can't seek: $!";
                push(@files, {
                        fh => $f,
                        queued => scalar <$f>,
                });
                $tempname++;
        }
}

# This is where you'd read the input file to exhaustion
# I'm just making up data.  The important part is the block itself.
while($_=rand() . "\n", $recs--) {
        push(@sortarr, $_);
        if (@sortarr==$max) {
                store(\@sortarr);
                @sortarr=();
        }
}
store(\@sortarr) if @sortarr;  # Store the leftovers

LOOP: {
        ($lowest)=(sort {
			sortsub($a->{queued}, $b->{queued});
			} grep(defined $_->{queued}, @files) )[0];
        last unless defined $lowest->{queued};
	
	# Do your processing here
        print $lowest->{queued};
	
        $lowest->{queued}=$lowest->{fh}->getline();
        redo;
}