#! perl
use strict;
use warnings;

while (<DATA>)
{
    s/__(\S+?)__/uc($1)/eg;
    print;
}

__DATA__
#ifdef __some_Long_name__
do stuff
#endif

##</code><code>##

18:25 >perl 1523_SoPW.pl
#ifdef SOME_LONG_NAME
do stuff
#endif

18:27 >