Interesting. Reading the context behind those SEARCH_EXTS definitions, it's in the Perl_find_script() function (aliased to macro find_script), and (to my reading) doesn't seem to be in the context of the -x ftest, that I can tell

Your .cmd search found (among others) the EU::MM instance where the four qw(.com .exe .bat .cmd) are shown, but that's in the EU::MM module, not in the -x ftest.

Some more searching for the ftest led me to pp_sys.c:PP(pp_ftrread) and the associated S_try_amagic_ftest() a few lines up. I eventually found the win32_fstat(), which I think calls the MSVCRT fstat(), so I don't think I can blame the port of perl for this: it looks like that might be due to the underlying library.

I've also verified that it is really just the four extensions (at least up to 3character extensions) that are considered "executable" to win32 perl:

use warnings;
use strict;

foreach my $ext ('a' .. 'zzz') {
    my $fn = 'x.'.$ext;
    open my $fh, '>', $fn or die "$fn $!";
    close $fh;
    printf STDERR "%-15s %d\n", $fn, -x $fn if -x $fn;
    unlink $fn;
}
__END__
x.bat           1
x.cmd           1
x.com           1
x.exe           1
[download]

I made an equivalent program in c using the sys/stat.h's _stat(), and it was the same four extensions, only. I guess this one really can be blamed on Microsoft. :-)