G'day symgryph,

Here's my take on a solution:

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

local ($/, $") = ("", ',');
my @fields = qw{user summary access group};
my $re = qr{ (?: ^user:\s(?<user>\S+)
             |   ^\s*summary\s"(?<summary>[^"]+)
             |   ^\s*access-level\s(?<access>\S+)
             |   ^\s*group\s(?<group>\S+) ) }mx;

while (<DATA>) {
    my %data;
    @data{keys %+} = values %+ while m{$re}g;
    say "@{[ map { $_ // '' } @data{@fields} ]}";
}   

__DATA__
user: myus44 [up] 
------------
 admin-state enabled 
 summary "Johnny Cash" 
 access-level group-defined 
 group mi-group  [up]

user: jar1543 [up] 
------------
 admin-state enabled 
 summary "Lara Croft" 
 access-level group-defined 
 group jar-head  [up]

user: myprivilegeduser [up]
-----------
admin-state enabled
access-level privileged
[download]

Output:

$ pm_1047505_regex.pl
myus44,Johnny Cash,group-defined,mi-group
jar1543,Lara Croft,group-defined,jar-head
myprivilegeduser,,privileged,
[download]

While this produces the output you indicated, you may want to revisit your requirements. ++Tux makes some very good points in this regard.