This does not work if you have other #ifdef blocks inside the DEBUG block. It will resume printing at the first #ifdef it encounters. Try it with this to see what I mean:

#include <stdio.h>

#ifdef DEBUG
  some code here
#ifdef SOLARIS
  solaris-specific code here
#endif /* SOLARIS */
  this is still debug code
#endif DEBUG
  this is normal code
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A possible solution would be to keep track of how many #ifs and #endifs you have seen. Something like this:

  my $state = 0;
  my $debug = 0;;

  while(<>) {
    $debug=1,$state++,next if /^\s*#if.*DEBUG/;
    $state++ if /^\s*#if/;
    $state-- if /^\s*#endif/;
    $debug=0,next if $debug && $state==0;
    print if $debug==0;
  }
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