my @todo = glob('d:/Profiles/sal/Desktop/testFolder/*.bld');  # some kind of file
my %done;
undef $/;
while(@todo) {
    my $file = shift @todo;
    print "File Name : $file\n";
    open FILE, "+<$file" or die "Can't open file $file: $!";
    $_ = <FILE>; # whole contents of file
    if(s/\bold\b/new/g) {
        # replace contents of file
        seek FILE, 0, 0;
        truncate FILE, 0;
        print FILE;
    }
    my($dir) = $file =~ m[(.*/)];
    while(/^#include (\S+);/mg) {
        push @todo, "$dir$1" unless $done{"$dir$1"}++;
    }
}

##</code><code>##

-hi.bld 
 -do.bld
  +one.bld