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

##</code><code>##

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 ];