If you used strict and warning, and fixed your code to avoid the first bunch of errors, then you'll get this set of errors:

c:\test>junk
syntax error at c:\test\junk.pl line 34, near "$OutputHandlePtr;"
Bareword "null" not allowed while "strict subs" in use at c:\test\junk
+.pl line 13.
Bareword "N" not allowed while "strict subs" in use at c:\test\junk.pl
+ line 18.
Bareword "P" not allowed while "strict subs" in use at c:\test\junk.pl
+ line 18.
Bareword "P" not allowed while "strict subs" in use at c:\test\junk.pl
+ line 18.
Bareword "N" not allowed while "strict subs" in use at c:\test\junk.pl
+ line 18.
Bareword "I" not allowed while "strict subs" in use at c:\test\junk.pl
+ line 18.
Bareword "null" not allowed while "strict subs" in use at c:\test\junk
+.pl line 32.
Bareword "N" not allowed while "strict subs" in use at c:\test\junk.pl
+ line 33.
Bareword "N" not allowed while "strict subs" in use at c:\test\junk.pl
+ line 33.
Bareword "P" not allowed while "strict subs" in use at c:\test\junk.pl
+ line 33.
Bareword "N" not allowed while "strict subs" in use at c:\test\junk.pl
+ line 33.
Execution of c:\test\junk.pl aborted due to compilation errors.
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And if you pay particular attention to the two 'Bareword "null"' messages, then you might get somewhere.

Also, it is often, but not always useful, to print $^E in a string context if the api call fails.

Update: If you are using AS perl, then take a look at the html pod for the package Win32, wher eyou will find that MessageBox() is available without using Win32::API.

Win32::MsgBox(MESSAGE [, FLAGS [, TITLE]])
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