This is a better match for what File::Spec is doing:

package XPlat;
use strict;
my $OS_TYPE = $^O eq 'MSWin32' ? 'win' :
              ($^O eq 'nonstop_kernel' ? 'nsk' : 'unix');
push @ISA, "XPlat::$OS_TYPE";

sub XPlat::win::fn  { print "windows version of function\n" }
sub XPlat::nsk::fn  { print "nsk version of function\n"     }
sub XPlat::unix::fn { print "unix version of function\n"    }

package main;

XPlat->fn(); # calls function in whichever package is appropriate
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As a bonus, you can use inheritance to share common methods by moving them up to a superclass, or produce other effects by creating additional subclasses.