Well this works for me on linux. I use ICEWM and the latest Xserver version. May say what Tk, distribution, Perl, and Xserver versions you have going.

Ugly.... but demonstrates it works.

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $types = [['Format Perl', '.pl']];

my $mw = new MainWindow ( ) ;

$mw->setPalette(background=>'black', foreground=>'white'); 

my @files;

$mw -> Button ( -text => 'Select File ' ,
                 -command => \&open_it
     )-> pack(-padx=>5, -pady=>5) ;
MainLoop ( ) ;

sub open_it {
    my $names = $mw ->getOpenFile ( 
                    -initialdir => '.',
                    -filetypes => $types,
                    -multiple => 1) ;
    
    foreach my $file( @{$names} ){
        print "Result ", $file, "\n" ;
       }
}
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