I have a log file ('dread-etc.log'). (...) My goal is to be able to run a perl script against this file so that it pulls out only the data that I have defined ("this is a bad error message"). (...) and write out a new log file called only_the_dread.txt.

There are many ways to do this... Here's a command line one-liner for the lazy:

  perl -ne "print if /this is a bad error message/" dread-etc.log >onl
+y_the_dread.txt
[download]

The part (pattern) in between the slashes is a regular expression, see perlre on how to craft them...

Type "perl -h" for a short description of what the options/switches (i.e. -ne) do — the long version is found in perlrun.

Also note that you might want to use single or double quotes, depending on which OS you're on (generally: single quotes on Unix, double quotes on Windows).

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(Update: quoted parts of the original question, as it has been deleted)