From Net::Telnet documentation:

 $prev = $obj->prompt($matchop);
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This method sets the pattern used to find a prompt in the input stream. It must be a string representing a valid perl pattern match operator. The methods login() and cmd() try to read until matching the prompt. They will fail with a time-out error if the pattern you've chosen doesn't match what the remote side sends.

With no argument this method returns the prompt set in the object. With an argument it sets the prompt to $matchop and returns the previous value.

The default prompt is '/[\$%#>] $/'

Always use single quotes, instead of double quotes