This seems to do basically what you want. I just extract what's between quotes and join them together with a backslash. Updated to handle "Account Deleted".
use strict;
use warnings;
my $string = q/ Win32_Account.Domain="ADDomain",Name="aduser1"/;
for my $string (q/ Win32_Account.Domain="ADDomain",Name="aduser1"/,
q/ Win32_Account.Domain="ADDomain",Name=""/) {
my @fields = $string =~ /"(.*?)"/g;
printf "New string is (%s)\n", $fields[1] eq '' ? 'Account deleted'
+: join '\\', @fields;
}
Caution: Contents may have been coded under pressure.