Here's how I would do it:

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

open FILE, "users.merit" or die "can't open users.merit.";

my $fieldreg = qr/([^(?:\s=\s)]+)\s=\s([^,]+)/;
my %users;
my $user;

while(<FILE>) {
    chomp;

    # next user?
    if(/(\S+)\s+$fieldreg/) {
        $user = $1;
        $users{$user}{$2} = $3;
        next;
    }
    if(/$fieldreg/) {
        $users{$user}{$1} = $2;
    }
}
print Dumper(\%users);
[download]

$0.02 :)