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my $nmbr = $mw->Entry(-width => 10, -text => 10);

 #!/usr/bin/perl -w
    use Tk;
...
        (my $value) = @_;
        print "Printing number $value\n";
    }

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

    sub do_randomise
    {
        (my $value) = @_;
        $$value = int rand 100;
    }