Please note how readable and maintainable the regexes become now!

This solves AnomalousMonk's test case here but is easily adaptable to various interpretations.

(I disagree in the case of unbalanced quotes, I'd rather ignore them. For this to happen drop the $-branch commented with "EOL".)

use v5.12;
use warnings;
use Test::More;

my $escaped = qr/\\./;

my $quoted  =
  qr/
        (['"])                          # --- start-quote

        (?:                             # --- inside 
            $escaped                    #     any escape-pair
        |
            .                           #     anything else
        )*?                             # non-greedy 

        (?:                             # --- end
            \g{-1}                      #   same quote
        |
            $                           #   EOL ends missing pair
        )
    /x;

my $re =
  qr/
        (?:
            $escaped                    #   any escape pair
        |
            $quoted                     #   any quoted string
        |
            \S                          #   any none whitespace
        )+                              # at least once
    /x;


my $str = q{This "is so" very simple.};

my @tests = (
             #  q{all '- and "-quotes properly balanced},
             [ q{This is simple.},           [ q{This}, q{is}, q{simpl
+e.}               ] ],
             [ q{     This  is   simple. },  [ q{This}, q{is}, q{simpl
+e.}               ] ],
             [ q{This is "so very simple".}, [ q{This}, q{is}, q{"so v
+ery simple".}     ] ],
             [ q{This "is so" very simple.}, [ q{This}, q{"is so"}, q{
+very}, q{simple.} ] ],
             [ q{This 'isn\'t nice.'},       [ q{This}, q{'isn\'t nice
+.'}               ] ],
             [ q{This "isn\"t nice."},       [ q{This}, q{"isn\"t nice
+."}               ] ],
             [ q{This 'isn\\\\'t nice.'},    [ q{This}, q{'isn\\\\'t},
+ q{nice.'}        ] ],
             [ q{This "isn\\\\"t nice."},    [ q{This}, q{"isn\\\\"t},
+ q{nice."}        ] ],
             [ q{This 'is not unnice.'},     [ q{This}, q{'is not unni
+ce.'}             ] ],
             [ q{This "is not unnice."},     [ q{This}, q{"is not unni
+ce."}             ] ],
             [ q{a "bb cc" d},               [ q{a}, q{"bb cc"}, q{d} 
+                  ] ],

             #  q{UNbalanced '- and "-quotes at absolute end of string
+},
             [ q{This is "so very simple}, [ q{This}, q{is}, q{"so ver
+y simple} ] ],
             [ q{This 'isn\'t nice.},      [ q{This}, q{'isn\'t nice.}
+          ] ],
             [ q{This "isn\"t nice.},      [ q{This}, q{"isn\"t nice.}
+          ] ],
             [ q{This 'isn\\\\'t nice.},   [ q{This}, q{'isn\\\\'t}, q
+{nice.}   ] ],
             [ q{This "isn\\\\"t nice.},   [ q{This}, q{"isn\\\\"t}, q
+{nice.}   ] ],
             [ q{This 'is not unnice.},    [ q{This}, q{'is not unnice
+.}        ] ],
             [ q{This "is not unnice.},    [ q{This}, q{"is not unnice
+.}        ] ],

             #  'what about these questionable cases?',
             [ q{is this"really so"simple now?}, [ q{is}, q{this"reall
+y so"simple}, q{now?} ] ],
             [ q{is this"really so"       now?}, [ q{is}, q{this"reall
+y so"},       q{now?} ] ],
             [ q{is     "really so"simple now?}, [ q{is}, q{"really so
+"simple},     q{now?} ] ],
             [ q{is this'really so'simple now?}, [ q{is}, q{this'reall
+y so'simple}, q{now?} ] ],
             [ q{is this'really so'       now?}, [ q{is}, q{this'reall
+y so'},       q{now?} ] ],
             [ q{is     'really so'simple now?}, [ q{is}, q{'really so
+'simple},     q{now?} ] ],

            );

plan tests => 0+@tests;

for my $test (@tests) {
    my ($str, $exp) = @$test;

    my $got;
    push @$got, $&
      while ($str =~ /$re/g);

    is_deeply($got, $exp, qq{<$str>: } . join('|', @$exp));
}
[download]

-*- mode: compilation; default-directory: "d:/tmp/pm/" -*-
Compilation started at Sun Oct 24 14:00:21

C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/break_not_quoted.pl 
1..24
ok 1 - <This is simple.>: This|is|simple.
ok 2 - <     This  is   simple. >: This|is|simple.
ok 3 - <This is "so very simple".>: This|is|"so very simple".
ok 4 - <This "is so" very simple.>: This|"is so"|very|simple.
ok 5 - <This 'isn\'t nice.'>: This|'isn\'t nice.'
ok 6 - <This "isn\"t nice.">: This|"isn\"t nice."
ok 7 - <This 'isn\\'t nice.'>: This|'isn\\'t|nice.'
ok 8 - <This "isn\\"t nice.">: This|"isn\\"t|nice."
ok 9 - <This 'is not unnice.'>: This|'is not unnice.'
ok 10 - <This "is not unnice.">: This|"is not unnice."
ok 11 - <a "bb cc" d>: a|"bb cc"|d
ok 12 - <This is "so very simple>: This|is|"so very simple
ok 13 - <This 'isn\'t nice.>: This|'isn\'t nice.
ok 14 - <This "isn\"t nice.>: This|"isn\"t nice.
ok 15 - <This 'isn\\'t nice.>: This|'isn\\'t|nice.
ok 16 - <This "isn\\"t nice.>: This|"isn\\"t|nice.
ok 17 - <This 'is not unnice.>: This|'is not unnice.
ok 18 - <This "is not unnice.>: This|"is not unnice.
ok 19 - <is this"really so"simple now?>: is|this"really so"simple|now?
ok 20 - <is this"really so"       now?>: is|this"really so"|now?
ok 21 - <is     "really so"simple now?>: is|"really so"simple|now?
ok 22 - <is this'really so'simple now?>: is|this'really so'simple|now?
ok 23 - <is this'really so'       now?>: is|this'really so'|now?
ok 24 - <is     'really so'simple now?>: is|'really so'simple|now?

Compilation finished at Sun Oct 24 14:00:21
[download]