$_ = "test;a  <>amp;kap;&da;ma; &amp alfa&romeo & mich;";

while ( s/([&][^ ]*);(?!( |&|$))/\1AMPSEMICOLON/ ) {};
s/AMPSEMICOLON/;\&/g;

# Output:
test;a  &<>&&kap;&da;&ma; &amp alfa&romeo & mich;