G'day astronogun,

While there's quite a few issues with the code you posted, I see nothing that would prevent appending to a text file:

$ ls -l input.txt
ls: input.txt: No such file or directory

$ perl -e '$input = q{input.txt}; @output = system ("ls -l >> input.tx
+t");'

$ ls -l input.txt
-rw-r--r--  1 ken  staff  16393 12 Mar 15:34 input.txt
[download]

Here's some of the issues I found with your code:

• Unless you have a very good reason not to, always put "use strict;" and "use warnings;" at the start of your code. See strict and warnings.
• Use meaningful names. You've called your output file input.txt and assigned that to a variable called $input. You also don't use the variable $input: I'm wondering if that was just an oversight or is input.txt actually an input file!
• system does not return an array but you're assigning its return value to @output. Again, I'm left wondering if this was just a poor naming choice coupled with a failure to check the documentation or are you actually expecting output from your java ... command to be placed in @output.
• You really need check return values. system gives a good description of how to go about this; it also mentions use of the autodie pragma.

I'd suggest something like the following code as a starting point from which you can inspect return values and investigate your problem further.

$ perl -e '
    use strict;
    use warnings;
    use autodie qw{:all};

    my $output_file = q{output.txt};
    my $return_value = system "ls -l >> $output_file";
'
[download]

You'll note both here, and earlier, that I've used a simple ls -l command to test the code. I'd suggest you do the same, to check whatever file-appending problem you're experiencing, before trying with "java -jar cmdline-jmxclient-0.10.3.jar - localhost:1100 com.ogs.red5.client.manager:name=Manager getConnectedAgents" which may have its own issues that could be masking your reported problem.