use strict;
use warnings;


sub dequote {
   my ($s) = @_;
   if ($s =~ /^"/) {
      $s =~ s/^"//;
      $s =~ s/"$//;
   } else {
      $s =~ s/\\([\\,])/$1/g;
   }
   return $s;
}


sub parse {
   my @terms;

   for ($_[0]) {
      my $key = /\G ( [^=,]+ ) = /xgc ? $1 : undef;

      my $val = /\G
         (
            " [^"]+ "
         |
            (?: [^\\,] | \\[\\,] )*
         )
      /xgc && dequote("$1");

      push @terms, [ $key, $val ];

      /\G $ /xgc
         and last;

      /\G , /xgc
         or die("Expected comma at pos ", pos(), "\n");

      redo;
   }

   return @terms;
}


# ---


use Data::Dumper qw( Dumper );

my $str = join ',', <<'__EOI__' =~ /.+/g;
key=value\,value
key=\\
key="value,value"
value
__EOI__

my @terms = parse($str);

local $Data::Dumper::Indent = 1;
local $Data::Dumper::Terse  = 1;
local $Data::Dumper::Useqq  = 1;
print(Dumper(\@terms), "\n");