Hi haukex,

You might want to also try using a file handle opened on a reference to your string:

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

This appears to be faster than your other subs, including the one with split (note that I had to change the count value for the result to be meaningful):

$ perl bench_split.pl
           s/iter      regex      index      split filehandle
regex        3.44         --        -9%       -36%       -48%
index        3.13        10%         --       -29%       -43%
split        2.21        56%        42%         --       -19%
filehandle   1.78        93%        76%        24%         --
[download]