That's not splitting what you do. You're also replacing escaped escapes with the escape (it seems you're replacing every !! with !). You cannot do that with just a split, you'll have to parse. I think this does what you want:

#!/usr/bin/perl

use 5.010;

use strict;
use warnings;
while (<DATA>) {
    chomp;
    my @a;
    my $i = 0;
    while (/(!.|.)/g) {
        my $char = $1;
        if ($char eq ";") {
            $i++;
            next;
        }
        $char = $1 if $char =~ /!(.)/;
        $a[$i] .= $char;
    }
    say "@a";
}


__DATA__
foo;bar
foo!;bar;baz
foo!!;bar!;baz;qux
foo!!!;bar!!;baz!;qux;quux
foo!!!!;bar!!!;baz!!;qux!;quux;garply
foo!!!!!;bar!!!!;baz!!!;qux!!;quux!;garply;waldo
[download]

Output:

foo bar
foo;bar baz
foo! bar;baz qux
foo!;bar! baz;qux quux
foo!! bar!;baz! qux;quux garply
foo!!;bar!! baz!;qux! quux;garply waldo
[download]