Both of these lines:

$socket->getline;
my $output = <$socket>;
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...read 'line oriented' input, i.e. they return when they read a newline from the socket. So if you want to read all the lines from the socket, you can use one of these:

while  ( defined( my $line = $socket->getline ) ) {

}

while (my $line = <$socket>) {

}

my @lines = <$socket>;
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The script either hangs or it just returns the "Welcome to device 'devicename', please input your commands".

Well, then it sounds like you should start your program by reading a line at a time from the socket--until the line you read matches that text, then send your command. Then read some more lines for the response.