my $re = qr{
  (?>      # don't backtrack into this group
     !!    # either the escape character,
           # followed by an escape character
     |     # or
     !;    # escape followed by delimiter char
     |     # or
   [^;\n]  # a character that is neither delimiter
           # character or a newline
  )+
}x;

while(<>) {
    chomp;
    $str = $_;
    print "$_\n";
    while ($str =~ /($re)/g) {
        print "  Chunk '$1' => ";
        $s = $1;
        $s =~ s/!!(?=(!|;))/!/g;
        print "$s\n";
    }
}