... but it gives me the problem that the variable message at the end gets split up into an indeterminate number of csv values ...

No, the third parameter to split limits the number of resultant fields so the $remainder scalar variable holds the entirety of your variable message.

Thats great and thank you very much for your help. I have one further issue though. My existing code now looks like this

#!/usr/bin/perl

use 5.010;
use strict;
use warnings;

while (my $line = <STDIN>) {
chomp($line);
my ( $mon, $day, $time, $dom, $login, $remainder ) =
   split m{:?\s+}, $line, 6;
my %monthNos = do {
   my $no = 0;
   map { $_ => ++ $no }
   qw{ Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec };
   };

my $yr = q{2012};
my $csv = sprintf q{%02d/%02d/%s %s,%s,%s,"%s"},
   $day, $monthNos{ $mon }, $yr, $time, $dom, $login, $remainder;

say $csv;
}
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As I wanted the remainder to be in double quotes so it would read in as one field. The problem is that remainder sometimes already contains double quotes which I would like to remove or replace with single quotes

I tried the following but it leaves be with an empty set of double quotes

my $remainder =~ s/""\"\""/\'/g;

Sorry to bother anyone with such trivial stuff, I played and found the the following worked

$remainder =~ tr/"/'/d;

Many Thanks