I have very little experience with Tk, but the docs (and the test I ran using your example code) indicate that defaults can be provided by simply adding -textvariable => \$defaultvalue to the list of options when you create your Entry object. Note that the -textvariable parameter must take a reference.

    my $default = 5;
    my $nmbr = $mw->Entry(-width => 10, -textvariable => \$default);
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