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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You already had the answer in your Label and Button widgets. Use -text => value for the Entry widget:

my $nmbr = $mw->Entry(-width => 10, -text => 10);
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The -textvariable option bobf mentioned binds a variable to the widget's value. You could do it that way and should if you want to access the value without using get. Consider:

 #!/usr/bin/perl -w
    use Tk;
    use strict;    
   
    my $mw = MainWindow->new;
    my $value = 10;
    $mw->Label(-text => 'Number')->pack;
    my $nmbr = $mw->Entry(-width => 10, -textvariable => \$value);
    $nmbr->pack;  

   
    $mw->Button(
        -text => 'Print',
        -command => sub{do_print($value);}
        )->pack;  
         
   
    MainLoop;      
   
   
    sub do_print
    {
        (my $value) = @_;
        print "Printing number $value\n";
    }
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Update: You might also like to try adding

    $mw->Button(
        -text => 'Randomise',
        -command => sub{do_randomise(\$value);}
        )->pack;
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before MainLoop and

    sub do_randomise
    {
        (my $value) = @_;
        $$value = int rand 100;
    }
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to the end of your program. Note that this time we pass a reference to the variable into the sub and can now update the value of the variable and thus the value shown in the Entry widget.