Are you sure you need perl for this? It sounds like all you need is the unix/linux/macosx "grep" utility (available for windows - just google "ms-windows grep")

grep critical *.txt
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That will show you every line from every *.txt file that contains the word "critical". If all you want is the list of file names, use the "-l" option:

grep -l critical *.txt
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If there's more you want to do, perl might be useful for that (or there might be another unix/linux/osx tool that does what you want...)