s/(abc="123")/$1 aabbcc/;

Just do something like this

my $string = q/aab"123"bcc abc="123" abc="345" aab"123"bcc/;
$string =~ s/(abc="123")/$1 aabbcc/;

# or better (doesn't do unnecessary replacing)
$string =~ s/(?<=abc="123")/ aabbcc/;

__END__
Benchmark: timing 300000 iterations of lookback, normal...
  lookback:  4 wallclock secs ( 4.32 usr +  0.01 sys =  4.33 CPU) @ 69
+348.13/s (n=300000)
    normal:  6 wallclock secs ( 6.01 usr +  0.02 sys =  6.03 CPU) @ 49
+759.50/s (n=300000)
            Rate   normal lookback
normal   49759/s       --     -28%
lookback 69348/s      39%       --
[download]

A suggestion for the next time, post some code (and the error message you get) so that we can see what you have so far ...

Update: AM is right, the positive lookahead was wrong. I replaced it with a lookbehind and now it works. To show the benefit I also included a small benchmark. As is always the case with benchmarks - results vary depending on the length of the text being replaced.

-- Hofmator

$string =~ s/(?=abc="123")/ aabbcc/;
does the same as:
$string =~ s/(abc="123")/ aabbcc$1/;
which is not correct.