perl -ne 'print if /^Header2:/ .. eof' testdata.txt

perl -0777 -ne '/^(Header2:.+)/ms and print $1' testdata.txt

perl -0777 -pe '$_=join q//,(split /^(Header2:)/m,$_,3)[1,2]' testdata.txt

perl -0777 -pe '$_=join q//,(split /^(?=Header2:)/m,$_,2)[1]' testdata.txt