After reading your post, I tried directly calling Win32::CopyFile(), and it worked with no warning:

use Win32;
Win32::CopyFile(
  "$src_path/$upc/$realtracknum.mp3",
  "$dst_path/$upc/$realtracknum.mp3",
  1
) or
  warn "couldnt File::Copy $src_path/$upc/$realtracknum.mp3: $!";
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And the actual call to Win32::CopyFile() in File/Copy.pm (nested within a large, unwieldy if..else chain) uses the same syntax:

return 0 unless @_ == 2;
return Win32::CopyFile(@_, 1);
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So the problem seems to be with File/Copy.pm calling Win32::CopyFile(). Whatever that seems to indicate, though, I'm not quite sure.

Addendum: I also tried passing an actual array, to see if that somehow pooched the XS code, but this still works without any warnings:

my @copy_args = (
  "$src_path/$upc/$realtracknum.mp3",
  "$dst_path/$upc/$realtracknum.mp3"
);
Win32::CopyFile(@copy_args, 1) or
  warn "couldnt File::Copy $src_path/$upc/$realtracknum.mp3: $!";
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