Do you really need to use split? Why not match exactly what you want in the main loop?

#!/usr/bin/perl

use strict;
use warnings;

use feature 'say';

while (<DATA>) {
  # should you need it, the value is in $2, undef if not present
  say $1 if m{^ \s* \#define \s+ (\S+) (?:\s+ (0|1))? \s* $}x;
}

__DATA__
  #define thread 1
  #define flag
  #define code 0
  #define const (100/5)
  #define flag_condn 1
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A couple of notes: if you are using an ancient (pre 5.10) perl, then don't use use feature 'say'; and say, instead use print $1, "\n";. The /x modifier allows nicer formating for the regexp: whitespace is ignored, comments are allowed (hence the \# here, since it's not a comment) and the regexp can be spread over several lines.