A simple solution to your problem doesn't involve perl at all. Using the shell you can achieve this in just a simple line:

$ sed 's/=/ /' complex_file | join -a 1 -1 1 -2 3 simple_file -
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This works only if you have the files sorted by the joint field, e.g:

$ mv simple_file simple_file.bk; sort simple_file.bk > simple_file
$ mv complex_file comple_file.bk; sed 's/=/\t/' complex_file.bk | sort
+ -k 3,3r | sed 's/\t/=/' > complex_file
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Hope this helps!

citromatik