My thinking is that could could modify your code to something like (untested):

#  assumes $filename is set from Tk 

my $fh;
Win32::LongPath::openL (\$fh, '<', $filename) 
  or die "Unable to open $filename";

my $parser   = Spreadsheet::ParseXLSX->new();
my $workbook = $parser->parse($fh);

#  do stuff
[download]

openL is documented at https://metacpan.org/pod/Win32::LongPath#openL-FILEHANDLEREF,MODE,PATH

You could also check the filename can be found using testL. https://metacpan.org/pod/Win32::LongPath#testL-TYPE,PATH

This all comes with the caveat that I have not used these functions, although if it does work then it will help solve one of my own longstanding headaches with a Gtk2 application.

Thank you for your idea. It works!?! Even for the path the module does not return a shortpath! How is this possible? I updated the script. It first tries to open it with the filehandle (on my test system it seems to work fine), it then tries to open it passing the shortpath. When no shortpath is returned by the module, the opration fails.

use strict;
use warnings;
use Tk;
use Win32::LongPath;
use Spreadsheet::ParseXLSX;

my $mw = Tk::MainWindow->new();
my $path;
my $button = $mw->Button(
    -text => "Select a file",
    -command => \&show_file_dialog,
)->pack(-side => 'left',);
my $label = $mw->Entry(
    -text => 'No file yet',
    -width => 100,
)->pack(-side => 'left',);
$mw->MainLoop();
sub show_file_dialog {
    my @ext = (
        ["Excel",    ['xlsx']],
        ["All files",           ['*']],
    );
    $path = $mw->getOpenFile(
        -filetypes => \@ext,
    );

    my $ShortPath = shortpathL ($path);
    $label->configure(-text => "$path - $ShortPath");
    
print "Trying to open file with Win32::LongPath::openL ... ";    
    my $fh;
    Win32::LongPath::openL (\$fh, '<', $path) 
      or die "Unable to open $path";

    my $parser   = Spreadsheet::ParseXLSX->new();
    my $workbook = $parser->parse($fh);
print "OK\n";    

print "Trying to open file with shortpathL path ... ";
    my $parser   = Spreadsheet::ParseXLSX->new();
    my $workbook = $parser->parse($ShortPath);
print "OK\n";
}
[download]

Good to hear it works.

openL is written to handle paths like the ones you are getting from the system. Spreadsheet::ParseXLSX just uses a plain open call when passed a file name, and this cannot handle some of the paths that are being passed (assuming your short paths retain some of the unicode characters).

You probably should not even need to convert the paths to short paths if you use openL to generate the file handle. That would simplify your code (if that's an issue).