Well, if you read the comments there, we start matching at foo. We match character by character (unless we get to the beginning of baz). Then, we must match bar.

Going through your string, we start at foo. Good. We check if we're at the beginning of baz - we aren't. Grab a character (' '). Still not at the beginning of baz, so we grab another character (' a'). Still not at the beginning of baz, so we grab another character (' a '). Now we're at the beginning of baz, so we obviously went too far - back off (' a'), and exit the "loop". Next we need to match 'bar' - but 'bar' isn't here. Thus we failed to match anything. (Actually, we'll back off the two characters that we did match in the complex expression, but that will still fail to find 'bar'.)

One rule of thumb is to always check if a match succeeds before using any of the special variables. So try something like this:

if (
  /foo       # Match starting at foo
   (?:       # Complex expression:
     (?!baz) #   make sure we're not at the beginning of baz
     .       #   accept any character
   )*        # any number of times
   bar       # and ending at bar
  /x
  )
{
  # print stuff out.
}
else
{
  print "Failed to match against '$_'\n";
}
[download]

use strict;
use warnings;
use YAPE::Regex::Explain;

my $regex = qr/foo       # Match starting at foo
 (?:       # Complex expression:
   (?!baz) #   make sure we're not at the beginning of baz
   .       #   accept any character
 )*        # any number of times
 bar       # and ending at bar
/x;


my $explanation = YAPE::Regex::Explain->new($regex)->explain;
print $explanation;
[download]

The regular expression:

(?x-ims:foo        # Match starting at foo
 (?:        # Complex expression:
   (?!baz)  # make sure we're not at the beginning of baz
   .        # accept any character
 )*         # any number of times
 bar        # and ending at bar
)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?x-ims:                 group, but do not capture (disregarding
                         whitespace and comments) (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n):
----------------------------------------------------------------------
  foo                      'foo'
----------------------------------------------------------------------
  (?:                      group, but do not capture (0 or more times
                           (matching the most amount possible)):
----------------------------------------------------------------------
    (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
      baz                      'baz'
----------------------------------------------------------------------
    )                        end of look-ahead
----------------------------------------------------------------------
    .                        any character except \n
----------------------------------------------------------------------
  )*                       end of grouping
----------------------------------------------------------------------
  bar                      'bar'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

Part of the problem might be that there's no capturing parens in that regex; (?:) marks a non-capturing grouping.