If your description is the entire problem, I would suggest using sed instead of Perl because it has syntax for dealing with exactly this kind of operation.

sed -n "/This is the temp line/,$"p infile > outfile
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I'm not hating on Perl by any means, I just use the simplest tool for the job. Of course if you have to do anything more than a regex or two with the text before generating the output, stick with Perl.