Lets see

http://search.cpan.org/grep?cpanid=ROBERTMAY&release=Win32-GUI-1.06&string=Clear&i=1&n=1&C=1

samples/Notepad.pl

46-    " > &Delete"           => { -name => "Delete",
47:                                -onClick => sub { my $self = shift;
+ $self->Edit->Clear(); 0; }
48-                              },
[download]

Right, notepad won't delete text unless you select text.

You need  ->SelectAll;

This would have been easier/faster to answer if your code was runnable. Short, Self Contained, Correct Examples make for effective questions