...
use if ($^O =~ m/MSWin32/i || ''), 'Win32';
...
my $is_win32 = $^O =~ m/MSWIN32/i;
...
sub test_file
{
  my $file = ucx(shift);
  ...
}

sub ucx
{
  my $fn = shift;
  return $is_win32 ? Win32::GetANSIPathName($fn) : decode('utf8', $fn);
}