#!/cadappl/bin/perl -w -Ilib

use strict;
use Data::Dumper;

my @r = ( 
  [ 5, 3, 5, 5 ],
  [ 1, 1, 2, 1 ],
  [ 2, 1, 2, 3 ],
  [ 5, 5, 1, 5 ],
  [ 1, 5, 1, 1 ],
  [ 2, 3, 5, 3 ]
);

my %lineTo;
foreach my $ref (@r)
{
        $lineTo{"$$ref[0],$$ref[1]"} = "$$ref[2],$$ref[3]";
}
my $start = "1,1";
my $curr = $start;
print "$curr\n";
while (($curr = $lineTo{$curr}) ne $start)
{
        print "$curr\n";
}
[download]

1,1
2,1
2,3
5,3
5,5
1,5
[download]