That would be: 
split /(?:$key_pat=)?$val_pat\K,/

[download]

Test: 

use 5.010;  # /\K/

use Data::Dumper;

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

my $key_pat = qr/[^=,]+/;
my $val_pat = qr/"[^"]+"|(?:[^\\,]|\\[\\,])*/;

print Dumper [ split /(?:$key_pat=)?$val_pat\K,/, $str ];

[download]

Using split means everything has to be parsed twice. Once to find the commas on which to split and once to find the composing terms.

Update: Simplified pattern slightly.

In reply to Re^3: 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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