You can create a font in your Tk application, for a later use:

$mono_font = $mw->fontCreate('mono', -family => 'Courier', -size => 10
+);
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After you can use it by name mono or by using the variable $mono_font

$mw->Label(-text => "test monospaced font", -font => 'mono');
$mw->Label(-text => "test monospaced font bis", -font => $mono_font);
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If you want all object of some class, or all widgets in the program to have the same font (or color, or size..) you can use the "options database" Tk uses:

$mw->optionAdd('*font', 'Courier 12');
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L*

PS

You can specify -font directly int Text or ROText object, you'll anyway need a padding tecnique:

#pay attention to win32 doublequotes!

perl -MTk -e "$mw=tkinit;$t=$mw->Text(-font=>'Courier')->pack;$t->inse
+rt('end', $_.(' ' x (10 - length $_)).qq(xxx\n)) for qw (aaaa bbb ccc
+ccc);MainLoop"
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PPS see Tk font size for ALL widgets

Thanks for your great support! now I fix the issue by your method. everything works!

Evel:

I think the anonymous poster above meant change to a monospaced font, which will make the characters all take the same width:

iiiiiiiii

Where a typical variable pitch font lets normally-narrow character take less horizontal space than a normally-wide character:

iiiiiiiii

wwwwwwwww

...roboticus

When your only tool is a hammer, all problems look like your thumb.

as an example, with Tkx i do :

  Tkx::font_configure('TkFixedFont',-size=>8);

.... 

  my $lb = $w_bfrm->new_tk__listbox( -height    =>3
                                   , -selectmode=>'extended'
                                   , -font      =>'TkFixedFont'
                                   );
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I using Tk, anyway thanks for your input.

Thanks, I just fix it by replacing the original default font with mono font. and curiosity I also want to know what the default font style name is?

G'day Evel,

"... I also want to know what the default font style name is?"

You can use the cget() method (see Tk::options) to determine the default option value of any widget.

You can find values quickly using a one-liner, e.g.

$ perl -E 'use Tk; say MainWindow->new->Listbox->cget("-background")'
#d9d9d9
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Some option values aren't individual scalars: you may get a reference returned, e.g.

$ perl -E 'use Tk; say MainWindow->new->Listbox->cget("-font")'
Tk::Font=SCALAR(0x7f972e1bbd80)
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You can just dereference as normal. Here's two examples showing the older "${ ... }", and newer "...->$*", syntaxes. See "perlref: Postfix Dereference Syntax" if you're unfamiliar with the second one.

$ perl -E 'use Tk; say ${ MainWindow->new->Listbox->cget("-font") }'
Helvetica -12 bold
$ perl -E 'use Tk; say MainWindow->new->Listbox->cget("-font")->$*'
Helvetica -12 bold
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Here's another example just to show different widgets have different defaults.

$ perl -E 'use Tk; say MainWindow->new->Text->cget("-font")->$*'
Courier -12
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Also be aware that, while Tk has its own defaults, these may be altered externally (e.g. a .Xdefaults file); or from within your application by a module you've loaded (e.g. a custom widget), or by your own code (e.g. one of the "option*()" methods). See Tk::option for details.

— Ken