It depends on what you want to do. If you want it to read only one line and then proceed, use

 ${ $log_msg } = <STDIN>;
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That will read just one line. If you instead want to do something like "read until I enter a null line", use a while loop, read one line at a time, and look for an empty one (only contains "\n").

For the second option, try File::Temp.

($fh, $filename) = tempfile($template, UNLINK => 1);
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should work for you. That will automatically unlink the file when the program ends. If you don't want the filehandle (though I think you do), just use undef as the first item in the list being assigned to. Since you're going to write to the file, I'd suggest just using the handy already-opened-for-write filehandle that the call to tmpfile returns.