IOW, you wish to split iff there are an even number of quotes since the start of string.

/(?<^[^"]*("[^"]*"[^"])*) / does not work: perl states that

Variable length lookbehind not implemented before HERE mark in regex

lookahead works, but. and the following code:

my @a = (
        'simple',
        'keep simple',
        'a "bit more" difficult',
        'an "increasing more" "and more" here');

foreach $_ (@a)
{
        s/ (?=("[^"]*"[^"])*[^"]+$)/ | /g;
        print "$_\n";
}
[download]

produces

simple
keep | simple
a | "bit more" | difficult
an | "increasing more" | "and more" | here
[download]

unfortunately, the regex "confuses" split and it's not usable, at least i was not able to. but why? :)