geometry does it:

$main->geometry ('500x500');

[download]

but I have to give a height as well. Can I change just the width?

Yes, by:

my $height = $main->reqheight; # or $main->height
$main->geometry ('500x' . $height);

[download]
Update: supply answer
Perl is Huffman encoded by design.

In reply to Re: How to set main window size in Tk by GrandFather
in thread How to set main window size in Tk by GrandFather

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