#!/usr/bin/perl -w
use strict;

sub tsort
{
    my( %pred, %succ );
    for my $av (  @_  ) {
        for my $i (  1 .. $#$av  ) {
            $pred{$av->[$i]}{$av->[$i-1]} ||= 1;
            $succ{$av->[$i-1]}{$av->[$i]} ||= 1;
        }
        $succ{$av->[-1]} ||= {};
    }
    my @output;
    while(  %succ  ) {
        my( $best, $count );
        for my $item (  keys(%succ)  ) {
            my $preds= keys %{$pred{$item}};
            if(  ! defined($count)  ||  $preds < $count  ) {
                $best= $item;
                $count= $preds;
                last   if  0 == $count;
            }
        }
        warn "Data contains a cycle, breaking at $best.\n"
            if  0 < $count;
        push @output, $best;
        for my $succ (  keys %{$succ{$best}}  ) {
            delete $pred{$succ}{$best};
        }
        delete $succ{$best};
    }
    return wantarray ? @output : \@output;
}

my @array1= ( 1, 3, 4, 6 );
my @array2= ( 1, 2, 4, 6 );
my @array3= ( 1, 2, 3, 5 );
my @output= tsort( \( @array1, @array2, @array3 ) );
print "@output\n";
@output= tsort( [1,2,3,4], [1,3,2,4] );
print "@output\n";

##</code><code>##

1 2 3 4 5 6
Data contains a cycle, breaking at 2.
1 2 3 4