Your code is not quite equivalent. Note that if the unless fails, his code will fall through to set $array[$count] while yours won't. I think that's a bug in the OP's code.

And for the OP, I think that reversing a Boolean is better expressed as $quoted = !$quoted rather than $quoted = 1 - $quoted.

There's no need to be destructive with s///; all you need is to walk the pattern with //g, and there's no need to special-case the empty quotes. More efficiency can be gained by reading strings of non-quote, non-comma characters. The code becomes very clear, though the regex is a little complicated.

sub csv_split    {
   local $_ = shift || return undef;
   my @array = ();
   my $count = my $quoted = 0;
   while ( /([^,"]+|[,"])/g )    {
       if ($1 eq ',' and !$quoted) { $count++; }
       elsif ($1 eq '"')           { $quoted = !$quoted; }
       else                        { $array[$count] .= $1 }
   }
   @array;
}
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