You're reading your input line-by-line but you're trying to match across lines. That won't work.

The following should work, reading the input all at once (see perlrun):

perl -g -i -pe ’s/(?<=look)\n(?=ahead)/-/g;’ myfile.txt

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In reply to Re^2: Using Look-ahead and Look-behind by Corion
in thread Using Look-ahead and Look-behind by Roy Johnson

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