my $string    = "test test %ae, test %ue test, %oe";

my $find    = "%(ae|oe|ue)";
my $replace    = '#\1;';

# pq's Lorbeeren hinzugefuegt
$replace = quotemeta $replace;
$replace =~ s/\\\\(\d)/\$$1/g;

# meine Beerchen ;)
eval "\$string =~ s/$find/$replace/g;";