Building on kcott's approach (and his test cases and their underlying assumptions), here's a regex-based solution. I've added a few test cases of my own, but their validity is questionable because I don't fully understand perl-diddler's requirements. No attempt has been made to compare performance.

Win8 Strawberry 5.8.9.5 (32)  Sun 10/24/2021  3:14:25
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

use Test::More;
use Test::NoWarnings;

sub pp { local $" = '| |';  "|@{$_[0]}|"; }  # for output pretty-print
+ing

my @tests = (
  q{all '- and "-quotes properly balanced},
  [ q{This is simple.},           [ q{This}, q{is}, q{simple.}        
+       ] ],
  [ q{     This  is   simple. },  [ q{This}, q{is}, q{simple.}        
+       ] ],
  [ q{This is "so very simple".}, [ q{This}, q{is}, q{"so very simple"
+.}     ] ],
  [ q{This "is so" very simple.}, [ q{This}, q{"is so"}, q{very}, q{si
+mple.} ] ],
  [ 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."}      
+       ] ],
  [ 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 very 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"really so"simple
+}, q{now?} ] ],
  [ q{is this"really so"       now?}, [ q{is}, q{this"really 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'really so'simple
+}, q{now?} ] ],
  [ q{is this'really so'       now?}, [ q{is}, q{this'really so'},    
+   q{now?} ] ],
  [ q{is     'really so'simple now?}, [ q{is}, q{'really so'simple},  
+   q{now?} ] ],

  );

my @additional = qw(Test::NoWarnings);  # each of these adds 1 test
plan 'tests' => (scalar grep { ref eq 'ARRAY' } @tests)
              + @additional
              ;

# an escape \ escapes ANY character.
my $rx_dq = qr{ " [^\\"]* (?: \\. [^\\"]*)* (?: " | \z) }xms;
my $rx_sq = qr{ ' [^\\']* (?: \\. [^\\']*)* (?: ' | \z) }xms;
my $rx_q  = qr{ $rx_dq | $rx_sq }xms;

# match quoted or non-space substrings.  alt order critical!
# my $rx_extract = qr{ $rx_q \S* | \S+ }xms;  # for non-questionable c
+ases
my $rx_extract = qr{ [^'"\s]* $rx_q [^'"\s]* | \S+ }xms;

VECTOR:
for my $ar_vector (@tests) {

    if (not ref $ar_vector) {
        note $ar_vector;
        next VECTOR;
        }

    my ($string, $ar_expected) = @$ar_vector;

    my @got = $string =~ m{ $rx_extract }xmsg;

    is_deeply \@got, $ar_expected, "|$string| -> " . pp $ar_expected;

    }  # end for VECTOR

^Z
1..25
# all '- and "-quotes properly balanced
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|
# UNbalanced '- and "-quotes at absolute end of string
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.|
# what about these questionable cases?
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| |n
+ow?|
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| |n
+ow?|
ok 25 - no warnings
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