Finally got a moment away from child minding :). Here's a non-eval technique:

use warnings;
use strict;

my $udStr = "abcabcabc";
my $udSearch = '(a)';
my $udRep = '---$1---';

print "before: $udStr\n";
my $before = $udStr;
$udStr =~ s/$udSearch/$udRep/;
my @starts = @-;
my @ends = @+;

for (1..$#starts)
  {
  my $replace = substr $before, $starts[$_], $ends[$_] - $starts[$_];
  $udStr =~ s/\$$_(?=\D)/$replace/;
  }
print "after: $udStr\n";
[download]

Prints:

before: abcabcabc
after: ---a---bcabcabc
[download]

This still doesn't fix (?{...}) and (??{...}) in $user_defined_search, but those could be filtered.