I realize it's your first post, but you didn't cut&paste complete code. Always check your code after pasting. Your button commands for Open and Exit are missing.

But back to your problem. It works for me on Linux. Maybe on Windows, you need to create your font first, like this: (Notice you can refer to the font by object or 'tag-name'

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

my $mw = MainWindow->new(
    -title      => 'Testing Tk::Menu',
    -width      => 500,
    -height     => 300,
    -background => 'yellow',
    -name       => 'Demo',
);
$mw->minsize( 500, 300 );

my $bf = $mw->fontCreate('big',
   -family=>'courier',
   -weight=>'bold',
   -size=>int(-18*18/14));

my $menubar = $mw->Menu(
    -type => 'menubar',
     -font => $bf
);

$mw->configure( -menu      => $menubar );
#$menubar->configure( -font => "Verdana 14 bold" ); 

my $m1 = $menubar->cascade(
    -label     => 'File',
    -tearoff   => 0,
    -menuitems => [
        [
            Button   => 'New',
            #-font    => "Verdana 16 bold",            # OK 
            -font => 'big',
            -command => \&create_new_file
        ],
        [
            Button   => 'Open',
            -command => \&open_file
        ],
        [
            Button   => 'Exit',
            -command => sub {exit}
        ],
    ]
);

MainLoop;

sub create_new_file { print "create_new_file...\n"; }

sub open_file { print "open_file...\n"; }

__END__
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