...

xsubpp
-typemap "...lib\ExtUtils\typemap"
-typemap "...inline-c-mytypemap-null\mytypemap.txt"
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https://metacpan.org/pod/xsubpp I see

typemap typemap

    Indicates that a user-supplied typemap should take precedence over
+ the default typemaps. This option may be used multiple times, with t
+he last typemap having the highest precedence.
[download]

But it seems by using  unmapped     T_PTR you're getting the old T_PTR, not the new one, you can't redefine T_PTR, which makes sense

Actually it would seem that default typemap is clobbering your typemap definitions

#!/usr/bin/perl --
use strict;
use warnings;
use Path::Tiny qw/ path /;
use Inline C =>
    Config =>
        BUILD_NOISY => 1,
        CLEAN_AFTER_BUILD => 0,
        typemaps => path( __FILE__ )->realpath->parent->path('mytypema
+p.txt'),
        NAME => 'markong',
        FORCE_BUILD => 1, ## debuggery
;;;;;;;;;;;;;

use Inline 'C' => <<'CODE';


typedef unsigned char * unmapped;

unmapped foo(unmapped name) {
  if(name) printf("True\n");
  else printf("False\n");
  return(NULL);
}

unsigned char * bar() {
  return (foo(NULL));
}


CODE

use Devel::Peek qw/ Dump /;
foo(undef);
bar();
exit( 0 );

__END__
[download]

...
Finished Build Compile Stage

INPUT Too_PTR= at inline-c-mytypemap-null-11139712.pl line 46.
Use of uninitialized value in subroutine entry at inline-c-mytypemap-null-11139712.pl line 46.
False
OUTPUT Too_PTR= at inline-c-mytypemap-null-11139712.pl line 46.
False

## cannot redefine T_PTR INPUT/OUTPUT
#  unmapped     T_PTR
unmapped     Too_PTR

brokenmapped    T_PTR

# cannot redefine typemap for "unsigned char*"
unsigned char *        Foo_PTR

INPUT
T_PTR
    $var = BROKEN_INT2PTR($type,SvIV($arg))


Too_PTR
    STMT_START {
        Perl_warn(aTHX_ \"INPUT Too_PTR=%s\", $arg);
        $var = INT2PTR($type,SvIV($arg));
        
    } STMT_END ;

Foo_PTR
    STMT_START {
        Perl_warn(aTHX_ \"INPUT Foo_PTR=%s\", $arg);
        $var = INT2PTR($type,SvIV($arg));
        
    } STMT_END ;

OUTPUT

Too_PTR
    STMT_START {
        Perl_warn(aTHX_ \"OUTPUT Too_PTR=%s\", $arg);
        sv_setiv($arg, PTR2IV($var));
        
    } STMT_END ;


Foo_PTR
    STMT_START {
        Perl_warn(aTHX_ \"OUTPUT Foo_PTR=%s\", $arg);
        sv_setiv($arg, PTR2IV($var));
        
    } STMT_END ;

T_PTR
    sv_setiv($arg, BROKEN_PTR2IV($var));
[download]