Quote

"How can I indicate that my distribution only works on a particular operating system?"

While it isn't a very elegant solution, the recommend approach is to either die in the Makefile.PL or Build.PL (or BAIL_OUT in a test file) with one of the following messages:

No support for OS
OS unsupported

CPAN Testers tools will look for one of those phrases and will send an NA (Not Available) report for that platform.

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use 5.010001;
use strict;
use warnings;
use Test::More;

my bail_out = 'MSWin32';

BAIL_OUT( "No support for $bail_out" ) if $^O eq $bail_out;