Tanktalus provided an excellent decription of how the regular expression attempted to match your string. If you would like to experiment with the regex some more I recommend using YAPE::Regex::Explain to get english descriptions of how a given regular expression works. It's much easier to read than the output of use re 'debug'

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]

Outputs:

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]