A number of minor things, most of which are personal preference:

1. I tend to use statement modifiers where there would only otherwise be one small statement in a controlled block.
2. remove the trailing white space from the captured strings and require at least one of each character type in the regex to avoid some nasty edge cases
3. Avoid intermediate variables where the calculation is trivial (@val for example)
4. Use double quote interpolation instead of concatenation

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my %hash;

while (<DATA>) {
    (my $key, my $value) = /(\S+\s+\S+)\s+(\S+\s+\S+)/;

    $value = join ":", split " ", $value;
    $hash{$key} .= ", " if exists $hash{$key};
    $hash{$key} .= $value;
}

print "$_ $hash{$_}\n" for sort keys %hash;

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

Prints:

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

If the code changes take longer than the time saved, it's fast enough already.

---

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

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