Beware the wily corner case. I don't know if  '#ifdef ___xxx___' and the cases that follow it are pertinent to your application, but just something to think about:

c:\@Work\Perl\monks>perl -wMstrict -le
"my @ln = (
   '#ifdef __some__Long_name__',
   '__w_x__y_z__',
   '__aa_bb__cc___dd__ xxx __pp__qq__',
   '#ifdef ___xxx___',
   '#ifdef __xxx',
   '#ifdef xxx__',
   );
 ;;
 for my $s (@ln) {
   print qq{'$s'};
   $s =~ s{ \b __ (\w+?) __ \b }{\U$1}xmsg;
   print qq{'$s' \n};
   }
"
'#ifdef __some__Long_name__'
'#ifdef SOME__LONG_NAME'

'__w_x__y_z__'
'W_X__Y_Z'

'__aa_bb__cc___dd__ xxx __pp__qq__'
'AA_BB__CC___DD xxx PP__QQ'

'#ifdef ___xxx___'
'#ifdef _XXX_'

'#ifdef __xxx'
'#ifdef __xxx'

'#ifdef xxx__'
'#ifdef xxx__'
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