sub dequote {
    my $x = $_[0];
    $x =~ s/!(.)/$1/sg;
    return $x;
}

while(<>) {
    chomp;
    my @fields = map dequote($_), /\G((?:[^!;]+|!.)*)(?:;|\z)/sg;
    print "$_ => " . join( '|', @fields ) . "\n";
#     print "$_ => @fields\n";
}

##</code><code>##

aval!!!!;bval => aval!!|bval|
aval!!!!!;bval => aval!!;bval|
a!!val!!!!!;bval! =>
!a!!!val!!!!!;bval!! => a!val!!;bval!|
a!val!;bva!l; => aval;bval|
a!!val!!;;bv!!al;; => a!val!||bv!al||