To overcome the limitation with the quick in-place edit not working on matching the first-line you can remove the -p and do it yourself like: 
perl -i.bak -e 'undef $/; $_=<>; s/line1\nline1/new content/g; print' files ..

In reply to Re: How can I match a multi-line regexp? by wesal
in thread How can I match a multi-line regexp? by Anonymous Monk

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