Why it doesn't work is explained nicely by dave_the_m in a related post Binding TK events.
\&mysub creates a reference to the sub mysub; 
\&mysub(...)

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calls mysub, then creates a reference to the returned value
and by davidj in the same post
"When you want to pass arguments to a callbakc, specify an array reference, with the callback code reference as the first element and the callback arguments as the subsequent array elements" In other words: instead of 

$input->bind('<Return>', [\&mysub($arg)]);

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you need to have 
$input->bind('<Return>', [\&mysub, $arg]);

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Thus... what happens is instead of binding my_test to 'can', you are executing 'my_test' and binding the result to 'can'. This happens I guess in the main_loop.

UPDATE: Fixed format of quoted text

In reply to Re: Tk bind mouse down question by Sandy
in thread Tk bind mouse down question by Plankton

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