It looks like ++moritz has identified your problems.

You might find coding it like this, is a little more readable:

#!/usr/bin/env perl

use strict;
use warnings;

while (<DATA>) {
    my (undef, $def, $val) = split;
    print "$def\n" if ! $val or $val eq '1';
}

__DATA__
  #define thread 1
  #define flag
  #define code 0
  #define const (100/5)
  #define flag_condn 1
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Output:

$ pm_define_extract.pl
thread
flag
code
flag_condn
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Update: I changed the print line following moritz' comments below. It use to be:

    print "$def\n" if ! $val or $val =~ /^0|1$/;
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