Ehm, I benchmarked the code I gave on the example data he gave ofcourse..

my $t = q["string (12" or 1 foot or 1') into 6" MySQL varchar"];
$t =~ s/(?<=.)"(?=.)/""/g;
[download]

versus

my $t = q["string (12" or 1 foot or 1') into 6" MySQL varchar"];
substr($t, 1, -1) =~ s/"/""/g;
[download]

And the number of iterations is shown in the benchmark output (2**20 = 1048576)