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) @ 69348.13/s (n=300000)
    normal:  6 wallclock secs ( 6.01 usr +  0.02 sys =  6.03 CPU) @ 49759.50/s (n=300000)
            Rate   normal lookback
normal   49759/s       --     -28%
lookback 69348/s      39%       --