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;
}
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