s/(?:\S+\s+){6}\K(.+)/($x=$1)=~tr,(),,d;$x/e

This removes the dependence on Perl version 5.14 for the  /r modifier for the  tr/// operator, but does not satisfy the latest approximation to the real spec as given here. (Don'tcha just love receiving specs via drip-feed?) The problem is easily fixed with a  (\S+) vice  (.+) capture (still requires Perl version 5.10+):

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 print 'perl version: ', $];
 ;;
 my $rec =
   'Jun 12 10 mail (sender@sender.com) - (recip1@domain.com),(recip2@d
+omain.com) -1.889 25623, queued_as: B67837C0052 Subject goes here Sen
+der(sender@sender.com
)';
 print qq{'$rec'};
 ;;
 $rec =~ s/(?:\S+\s+){6}\K(\S+)/(my $x=$1)=~tr,(),,d;$x/e;
 print qq{'$rec'};
"
perl version: 5.010001
'Jun 12 10 mail (sender@sender.com) - (recip1@domain.com),(recip2@doma
+in.com) -1.889 25623, queued_as: B67837C0052 Subject goes here Sender
+(sender@sender.com)'
'Jun 12 10 mail (sender@sender.com) - recip1@domain.com,recip2@domain.
+com -1.889 25623, queued_as: B67837C0052 Subject goes here Sender(sen
+der@sender.com)'
[download]

That still leaves a dependence on version 5.10 for the  \K regex operator, again easily fixed with another capture:

c:\@Work\Perl\monks>perl -wMstrict -le
"print 'perl version: ', $];
 ;;
 my $rec =
   'Jun 12 10 mail (sender@sender.com) - (recip1@domain.com),(recip2@d
+omain.com) -1.889 25623, queued_as: B67837C0052 Subject goes here Sen
+der(sender@sender.com)';
 print qq{'$rec'};
 ;;
 $rec =~ s/((?:\S+\s+){6})(\S+)/(my $x=$2)=~tr,(),,d;$1.$x/e;
 print qq{'$rec'};
"
perl version: 5.008009
'Jun 12 10 mail (sender@sender.com) - (recip1@domain.com),(recip2@doma
+in.com) -1.889 25623, queued_as: B67837C0052 Subject goes here Sender
+(sender@sender.com)'
'Jun 12 10 mail (sender@sender.com) - recip1@domain.com,recip2@domain.
+com -1.889 25623, queued_as: B67837C0052 Subject goes here Sender(sen
+der@sender.com)'
[download]