For the case where it isn't known that [cow] follows [moo] you could also use the ... range operator to make the solution more general. The following prints the range from [moo] to the next line starting with [:

    perl -ne'print if /^\[moo\]/ ... /^\[/ and !/^\[/' log.txt
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The 3 dot operator ... works like the 2 dot operator .. except that the right operand isn't tested until the next evaluation.

--
John.