perl -e "qx(explorer.exe C:\Path\To\my.pdf)"
[download]

perl -e "qx'explorer.exe C:\Path\To\my.pdf'"
[download]

only difference is the qx delimiter. (insert smiley with gaping open mouth here)

From perlop qx:

Using single-quote as a delimiter protects the command from Perl's double-quote interpolation, passing it on to the shell instead:

perl -e "qx(explorer.exe C:\\Path\\To\\my.pdf)"

works

I finally got it work the way I want by using:

sub ShowPDF {
  my $pdf=shift;

  my $action = {
      MSWin32 => sub { $pdf=~s/\//\\/g; qx/explorer.exe "$url"/; },
      darwin  => sub { system qq{open "$pdf" >/dev/null 2>&1 &} },
  }->{$^O}    || sub { system qq{xdg-open "$pdf" >/dev/null 2>&1 &} };

  $action->();
}
[download]

I just had to add the RE to make sure there is only '\' in the path (its porting from Unices).

Using "explorer" command did the trick and solved my issue !

Many, many , thank to you all :-))

F.

Aaah! Yes, of course.
And explorer.exe seems to be one of the few programs who don't understand forward slashes in paths :-(