This was with version 5.14, using Cygwin. I will try with a more recent version under Windows.

Update This very strange: running the exact same program on the same computer but under a pure Windows setting and perl 5, version 24, subversion 1 (v5.24.1) and from a bash console, I obtain awfully bad results for the file handle solution:

           s/iter filehandle      regex      index      split
filehandle    100         --       -97%       -97%       -98%
regex        3.50      2758%         --       -14%       -42%
index        3.02      3217%        16%         --       -33%
split        2.02      4868%        74%        50%         --
[download]

I can only suspect that there might be something wrong with the management of Windows end-of-line pairs of characters.

Still under Windows, same program, from a dos windows, with version v5.16.3:

           s/iter      regex      index      split filehandle
regex        3.69         --        -2%       -29%       -39%
index        3.63         2%         --       -27%       -38%
split        2.63        40%        38%         --       -15%
filehandle   2.25        64%        61%        17%         --
[download]

I also tried (again on Cygwin with Perl version v5.14.4) this additional sub:

    filehandle2 => sub {
        open my $str_fh, "<", \$str or die "cannot open fh $!";
        my @lines = <$str_fh>;
        for (@lines) {
            chomp;
            s/o/i/g;
            ;
        }
     },
[download]

but the result is not so good as the first filehandle solution (but still better than the others):

$ perl bench_split.pl
            s/iter       regex       index       split filehandle2  fi
+lehandle
regex         3.44          --        -10%        -35%        -45%    
+    -48%
index         3.11         11%          --        -28%        -39%    
+    -42%
split         2.24         53%         39%          --        -16%    
+    -20%
filehandle2   1.89         82%         65%         19%          --    
+     -5%
filehandle    1.80         92%         73%         25%          5%    
+      --
[download]

Finally, I also tried this solution with a map:

    filehandle3 => sub {
        open my $str_fh, "<", \$str or die "cannot open fh $!";
        my @lines = map { chomp; s/o/i/g } <$str_fh>;
     },
[download]

and thought it might be faster, but this turns out to be slower than all the other solutions:

$ perl bench_split.pl
            s/iter filehandle3       regex       index       split  fi
+lehandle
filehandle3   3.64          --         -7%        -16%        -39%    
+    -51%
regex         3.38          8%          --         -9%        -34%    
+    -47%
index         3.07         18%         10%          --        -28%    
+    -42%
split         2.22         64%         52%         38%          --    
+    -20%
filehandle    1.77        105%         90%         73%         25%    
+      --
[download]