Update 1: Added walk2 by pushing to stack.
Update 2: The string length now obtained inside C.
Update 3: Changed from for loop to while loop.

Ah, I tried Inline::C. Walk1 runs faster than split in older Perl <= 5.20.x.

...

use Inline 'C' => <<'END_C';

  #include <stdlib.h>
  #include <string.h>

  SV * c_walk1(SV *sv)
  {
      STRLEN len;
      const char *s = SvPV_const(sv, len);
      const char *m = s, *strend = s + len;

      AV *ret = newAV();

      while (s < strend) {
          if (*s++ == '\n') {
              av_push(ret, newSVpvn(m, s - m));
              m = s;
          }
      }

      return newRV_noinc((SV *) ret);
  }

  void c_walk2(SV *sv)
  {
      STRLEN len;
      const char *s = SvPV_const(sv, len);
      const char *m = s, *strend = s + len;

      Inline_Stack_Vars;
      Inline_Stack_Reset;

      while (s < strend) {
          if (*s++ == '\n') {
              Inline_Stack_Push(sv_2mortal(newSVpvn(m, s - m)));
              m = s;
          }
      }

      Inline_Stack_Done;
  }

END_C

# walk1 - receive array ref
{
    my $string = $huge_string;
    my $start  = time;
    my $lines  = c_walk1($string);

    printf "duration walk1: %0.3f seconds\n", time - $start;
    printf "%d lines\n\n", scalar(@$lines);
}

# walk2 - receive list
{
    my $string = $huge_string;
    my $start  = time;
    my @lines  = c_walk2($string);

    printf "duration walk2: %0.3f seconds\n", time - $start;
    printf "%d lines\n\n", scalar(@lines);
}
[download]

Output from Perl 5.20.3 and Perl 5.26.1

$ /opt/perl-5.20.3/bin/perl demo.pl

duration memfh: 0.526 seconds
917504 lines

duration regex: 0.448 seconds
917504 lines

duration split: 0.110 seconds
917504 lines

duration trim : 0.268 seconds
917504 lines

duration walk1: 0.073 seconds
917504 lines

duration walk2: 0.110 seconds
917504 lines
[download]

$ /opt/perl-5.26.1/bin/perl demo.pl

duration memfh: 0.479 seconds
917504 lines

duration regex: 0.442 seconds
917504 lines

duration split: 0.066 seconds
917504 lines

duration trim : 0.259 seconds
917504 lines

duration walk1: 0.070 seconds
917504 lines

duration walk2: 0.084 seconds
917504 lines
[download]

Regards, Mario