while ( <> ) {
    chomp;
    s/!;/\x001/g;
    s/;/\x000/g;
    s/\x001/;/g;
    s/!!/!/g;
    @a = split /\x000/;
    print '{' . (join '}{', @a) . "}\n";
}
__END__
foo;bar;baz
fred!;flintstone;barney!!rubble
eggs!;spam!;toast!;spam;bacon!!;eggs!;spam!!toast!;spam;spamspam!!eggs!!;spam