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

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

##</code><code>##

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