Thanks, it was as simple as

my $can = $canvas; 
eval($buffer);
[download]

I did need to convert the name $canvas to $can.

While I was waiting for your reply, I also found that this worked

use Sub::Install;
Sub::Install::install_sub({  
       code => sub {eval $buffer}, 
        into => main::, 
        as   => 'foo'
   }); 

&foo($canvas);
[download]

Thanks again.