A1 abc yellow
B1 xyz green
A2 cde red
A1 abc green
A2 cde yellow
A1 abc blue

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

use Data::Table;

my $t = Data::Table::fromCSV('data.txt', 0, undef,{delimiter => ' '});
my $mt = $t->melt(['col1', 'col2']);
my $ct = $mt->cast(['col1', 'col2'], 'variable', Data::Table::STRING, 'value', \&col_join);

$ct->sort('col1', 1, 0);

print $ct->csv(0, {delimiter => " "});

exit;

sub col_join {
    my @data = @_;
    my $joined = join(":", @data);
    return($joined);
}

##</code><code>##

A1 abc yellow:green:blue
A2 cde red:yellow
B1 xyz green