One potential alternative, ensuring keys stay in the same order: 
#!/usr/bin/perl

use strict;
use warnings;

my @keys;
my %vals;

while (<DATA>) {
    chomp;
    my @data = split;
    my $key = join ' ', splice @data, 0, 2;
    push @keys, $key if !$vals{$key};
    push @{$vals{$key}}, join ':', @data;
}

for (@keys) {
    print "$_ " . join(', ', @{$vals{$_}}) . "\n";
}

__DATA__
id1    name1 cat1  catname1
id1    name1 cat2  catname2
id2    name2 cat3  catname3
id3    name3 cat1  catname1
id3    name3 cat4  catname4

[download]
Outputs: 
id1 name1 cat1:catname1, cat2:catname2
id2 name2 cat3:catname3
id3 name3 cat1:catname1, cat4:catname4

[download]
- Miller

In reply to Re: Appending values to existing hash key by wind
in thread Appending values to existing hash key by jaypal

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