Here's my take on the problem. It has the advantage, IMHO, of being more easily adaptable to changing requirements because it is more modular.

Notes:

• The regex uses  \x22 in place of  " (double-quote) to avoid Windoze command-line escape-ology.
• A quoted string cannot contain any sort of double-quote, escaped or otherwise.
• A parenthetic group cannot contain a  ')' (right-paren).

(Sorry for the line-wrap.)

>perl -wMstrict -le
"my $s = 'key1=val1 key2=val2 key3=val3 key4=\"val4a val4b\" '
       . 'key5=\"val5key=(0 1 2 3)\" key6=(val6a val6b)'
       ;
 ;;
 my $key   = qr{ [[:alpha:]] [[:alnum:]]+ }xms;
 my $val   = qr{ [[:alpha:]] [[:alnum:]]+ }xms;
 my $d_quo = qr{ \x22 [^\x22]* \x22 }xms;
 my $paren = qr{ [(]  [^)]*    [)]  }xms;
 ;;
 my $vals  = qr{ $val | $d_quo | $paren }xms;
 ;;
 my @opts = $s =~ m{ $key \s* = \s* $vals }xmsg;
 ;;
 print qq{'$s'};
 print qq{'$_'} for @opts;
"
'key1=val1 key2=val2 key3=val3 key4="val4a val4b" key5="val5key=(0 1 2
+ 3)" key6=(val6a val6b)'
'key1=val1'
'key2=val2'
'key3=val3'
'key4="val4a val4b"'
'key5="val5key=(0 1 2 3)"'
'key6=(val6a val6b)'
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