A general rule of thumb is that a negative lookaround can never work if there is a .* or any other variable-length "general" pattern next to it. In your case, you have two such things, .* to the right and [^\n]+ to the left of (?!\sLib\s). You can easily check how Perl matched your strings against the regular expression by printing out the match variables:

while (<DATA>) {
/
 (                               #Start capture
  (                              #Start Sub descript header
   '\*{20,}                      #start '------ delimiter
   (\n'[^\n]*?)+\n               #middle of header
   '\*{20,}\n                    #end '------ delimiter
  )?                             #End Sub header (optional)
  (Private\s|Public\s|Friend\s)? #Scope (optional)
  (Static\s)?                    #Static (optional)
  (Sub\s|Function\s)             #Sub or Function (mandatory)
  [^\n]+                         #more stuff on same line
  (?!\sLib\s)                    #but not if Lib on line
  .*                             #and the rest of the file
 )                               #End capture
/sx
and print "[$6/$7/$8]\n";
};
__DATA__
Private Declare Function DeleteFile Lib "kernel32" Alias "DeleteFileA"
+ _
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One approach to make your parser more robust is to make it more specific, like explicitly parsing out the function/sub name and expecting (or rather, denying) the Lib keyword immediately after the function name:

while (<DATA>) {
print;
/
 (                               #Start capture
  (                              #Start Sub descript header
   '\*{20,}                      #start '------ delimiter
   (\n'[^\n]*?)+\n               #middle of header
   '\*{20,}\n                    #end '------ delimiter
  )?                             #End Sub header (optional)
  (Private\s|Public\s|Friend\s)? #Scope (optional)
  (Static\s)?                    #Static (optional)
  (Sub\s|Function\s)             #Sub or Function (mandatory)
  (\w+)\s+                       # sub name
  ((?!Lib\s))                    #no Lib on line
  ([^\n]+)                       #more stuff on same line unless "Lib"
  (.*)                           #and the rest of the file
 )                               #End capture
/sx
    and print "[$6/$7/$8]";
};

__DATA__
Private Declare Function DeleteFile Lib "kernel32" Alias "DeleteFileA"
[download]

To be a bit more specific about the word "general" above, a negative lookahead will never work if there is a variable length quantifier next to it with a pattern that will also match (parts of) the phrase you want to avoid.

Thanks so much for your quick reply. Sorry it took me so long to get back. I had made a few mistakes, and it doesn't take much to screw up a RegEx. I put in a bit more detail, like capturing the fucntion name and parameters. A bit more screaming at it until a slash turned into a backslash, and it finally worked.

use strict;
undef $/;

while (<DATA>) {
print;
/
  (                               #Start capture
   (                              #Start Sub descript header
    '-{20,}                       #start '------ delimiter
    (\n'[^\n]*?)+\n               #middle of header
    '-{20,}\n                     #end '------ delimiter
   )?                             #End Sub header (optional)
   (Private\s|Public\s|Friend\s)? #Scope (optional)
   (Static\s)?                    #Static (optional)
   (Sub\s|Function\s)             #Sub or Function (mandatory)
   (\w+)                          #Sub name
   (                              #Start Params
    \(                            #Left paren
    ([^\)])*                      #Optional params inside
    \)                            #Right paren
   )?                             #End Params (optional)
   \s+                            #Space
  ((?!Lib\s))                     #no Lib on line
  ([^\n]+)                        #more stuff on same line unless "Lib
+"
   (.*)                           #and the rest of the file
  )                               #End capture
/sx;
};

__DATA__
Private thingy as String
Private Declare Function DeleteFile Lib "kernel32" Alias "DeleteFileA"

'-----------------------------------------------------------
' This is a header
'-----------------------------------------------------------
Private Function foo(byVal x as Long, byVal y as String) as Integer
[download]

It correctly ignores the first three lines and starts capturing at the '-----------. Thanks!

Matching a pattern that doesn't include another pattern

You might want to capture everything between foo and bar that doesn't include baz. The technique is to have the regex engine look-ahead at every character to ensure that it isn't the beginning of the undesired pattern:

/foo  # Match starting at foo
 (         # Capture
 (?:       # Complex expression:
   (?!baz) #   make sure we're not at the beginning of baz 
   .       #   accept any character
 )*        # any number of times
 )         # End capture
 bar  # and ending at bar
/x;
[download]

 (?:       # Complex expression:
   (?!Lib) #   make sure we're not at the beginning of Lib
   [^\n]       #   accept any character
 )+        # any positive number of times
[download]

Great tutorial! Thanks!