Loop through your input array, splitting on whitespace into numbers and descriptions. Stuff them into a Hash-of-Arrays data structure, with descriptions as keys and numbers as arrays. Then loop through the HoA, printing as desired:

use strict;
use warnings;

my @ins = (
'00001 Description1',
'00002 Description2',
'00003 Description1'
);

my %data;
for (@ins) {
    my ($num, $desc) = split;
    push @{ $data{$desc} }, $num;
}

for (sort keys %data) {
    print join("\t", @{ $data{$_} }), "\t$_\n";
}

__END__

00001    00003    Description1
00002    Description2
[download]

See also perldsc