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");

[download]

The above assumes slashes can also be escaped.
The above otherwise assumes everything you didn't specify is disallowed.

The following might be better to get the value, but differs from what you asked: 

sub dequote {
   my ($s) = @_;
   if ($s =~ /^"/) {
      $s =~ s/^"//;
      $s =~ s/"$//;
   }
   $s =~ s/\\(.)/$1/sg;
   return $s;
}

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

[download]

Update: Fixed to use double quotes instead of single quotes. Misread.
Update: Remove "or die("Expected value at pos ", pos(), "\n")" after the value extraction match. It'll never be executed since the pattern can match zero characters.

In reply to Re: Help with regex for complicated key=value string by ikegami
in thread Help with regex for complicated key=value string by mscharrer

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