You can use the flexibility of a regexp to match the expected prompt without specifying the exact string. That way you can avoid the $device interpolation. It also would be better if you could anchor the prompt as the last thing read using '$'. You should turn off paging so you don't have to deal with a "more" paging prompt.

$telnet->prompt('/\w+# $/');
$telnet->waitfor($telnet->prompt);
$telnet->cmd("terminal length 0");
@lines = $telnet->cmd("show run");
while (@lines) {
    print if /ip prefix-list/;
}
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