under Win32
perl test.pl  QUICK=BROWN\ FOX JUMPED=OVER\ THE\ LAZY DOG

produced an @ARGV of

$ARGV[0] = 'QUICK=BROWN\\';
$ARGV[1] = 'FOX';
$ARGV[2] = 'JUMPED=OVER\\';
$ARGV[3] = 'THE\\';
$ARGV[4] = 'LAZY';
$ARGV[5] = 'DOG';

So to achieve the result required I tried

my $line = join ' ', @ARGV;
$line =~ s/\s(?=\S+=)/\t/g;
@pairs = split(/\t/, $line);
%defaults = map {/([^=]+)           # everything up to the = sign into
+ $1
                       =            # the = sign
                       ((\S+\\\s)+  # all words followed by "\ "
                       \S+)         # word after the last "\ "
                       /x?($1=>$2):()} @pairs;

for ( keys %defaults) {
  print $_,"=",$defaults{$_},"\n";
};

which produced

QUICK=BROWN\ FOX
JUMPED=OVER\ THE\ LAZY
[download]

-_-_-_-_-_-_-_-_-_-_-_-
My memory concerns me
- but I forget why !!!
[download]