Hmmm...     This is the best I could do, but it's a bit icky for Anonymonk's OPed application:

>perl -wMstrict -le
"sub rx (&@) {
   return map { local $_ = $_;  ()= $_[0]->();  $_; } @_[1..$#_];
   }
 ;;
 my $s = 'a b c d';
 print rx { s/ /_/g } $s;
 ;;
 my @ra = ('p q r', 'x y z z y');
 print rx { s/ /_/g } @ra;
 ;;
 my $x =
   q{Here are [[a [[ variable]] number [[of ]words]] in brackets};
 print qq{'$x'};
 ;;
 $x =~ s( \[\[ (.*?) \]\] )( (rx {s/ /_/g} $1)[0] )xmsge;
 print qq{'$x'};
"
a_b_c_d
p_q_rx_y_z_z_y
'Here are [[a [[ variable]] number [[of ]words]] in brackets'
'Here are a_[[_variable number of_]words in brackets'
[download]

Bottom line:  s///r is a lot nicer. Thanks again to whoever thought of that one.