Eureka!

The trick is in the path. LabFrame does not have a .font property, but its subwidget of Label does: 
use warnings;
use strict;
use Tk;

my $mw = MainWindow->new();
$mw->geometry('200x200');

$mw->optionAdd('*font', 'helvr12');
$mw->optionAdd('*LabFrame.label.font', 'helvb12');

my $lab_frm = $mw->LabFrame(-label => 'LabFrame Test')->pack;
$lab_frm->Label(-text => 'Label Test')->pack;

print STDERR $lab_frm->Subwidget('label')->PathName, "\n";

MainLoop;

[download]
Update: Another interesting finding. This does not work with LabEntry because, from what I can tell, it does not advertise its Label subwidget. This can, however, be worked around by changing both subwidgets, then reverting Entry: 
$mw->optionAdd('*LabEntry*font', 'helvb12');
$mw->optionAdd('*LabEntry.entry.font', 'helvr12');

[download]

In reply to Re^4: Tk and optionAdd's scope by eff_i_g
in thread Tk and optionAdd's scope by eff_i_g

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.