If you want to do a quick in-place edit of a bunch of files, you can use this syntax:

perl -pi.bak -e 'undef $/; s/line1\nline2/new content/' files ..
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This will create ".bak" files for every file it operates on. NOTE that it won't find any matches on the first line. If someone knows another one-liner that also matches on the first line, I'm interested to see it. :)