sub s_index{
 my $pos = 0;
 my $cnt = 0;
 while (index($str, $sep, $pos)){
  $pos++;  
  $cnt++;
 }
 $cnt{s_index}=$cnt;
}

##</code><code>##

my $_s_index = sub {
  my $pos = 0;
  my $cnt = 0;
  local $[ = 1;  # ook!
  my $pos = -1;
  ++$cnt while $pos = index($dta,"\n\n",$pos+2);
  $cnt{_s_index}=$cnt;
};
 
my $_while_re = sub{
  my $cnt = 0;
  my $recsep = "\n\n";
  $cnt++ while $dta =~ /$recsep/g;
  $cnt{_while_re} = $cnt;
};
=>resulted in:
Benchmark: timing 300 iterations of m_array, m_array_M, m_for, m_for_M, m_while, m_while_M, s_index, tr, while_re...
   m_array: 11 wallclock secs ( 5.13 usr +  0.04 sys =  5.17 CPU) @ 58.03/s (n=300)
 m_array_M:  4 wallclock secs ( 1.57 usr +  0.02 sys =  1.59 CPU) @ 188.68/s (n=300)
     m_for:  9 wallclock secs ( 4.19 usr +  0.00 sys =  4.19 CPU) @ 71.60/s (n=300)
   m_for_M:  3 wallclock secs ( 1.41 usr +  0.01 sys =  1.42 CPU) @ 211.27/s (n=300)
   m_while:  5 wallclock secs ( 2.46 usr +  0.00 sys =  2.46 CPU) @ 121.95/s (n=300)
 m_while_M:  2 wallclock secs ( 0.94 usr +  0.00 sys =  0.94 CPU) @ 319.15/s (n=300)
   s_index:  2 wallclock secs ( 0.88 usr +  0.00 sys =  0.88 CPU) @ 340.91/s (n=300)
        tr:  1 wallclock secs ( 0.36 usr +  0.00 sys =  0.36 CPU) @ 833.33/s (n=300)
            (warning: too few iterations for a reliable count)
  while_re:  2 wallclock secs ( 0.90 usr +  0.00 sys =  0.90 CPU) @ 333.33/s (n=300)
 
=== check counts ===
         _while_re  2000
          _m_for_M  2000
        _m_array_M  2000
               _tr  6000
        _m_while_M  2000
          _m_while  6000
          _s_index  2000
            _m_for  6000
          _m_array  6000

##</code><code>##

Benchmark: timing 300 iterations of m_array, m_array_M, m_for, m_for_M, m_while, m_while_M, s_index, tr, while_re...
   m_array: 12 wallclock secs ( 5.04 usr +  0.04 sys =  5.08 CPU) @ 59.06/s (n=300)
 m_array_M: 11 wallclock secs ( 5.08 usr +  0.03 sys =  5.11 CPU) @ 58.71/s (n=300)
     m_for:  9 wallclock secs ( 4.19 usr +  0.00 sys =  4.19 CPU) @ 71.60/s (n=300)
   m_for_M:  8 wallclock secs ( 4.10 usr +  0.00 sys =  4.10 CPU) @ 73.17/s (n=300)
   m_while:  5 wallclock secs ( 2.26 usr +  0.02 sys =  2.28 CPU) @ 131.58/s (n=300)
 m_while_M:  5 wallclock secs ( 2.28 usr +  0.02 sys =  2.30 CPU) @ 130.43/s (n=300)
   s_index:  5 wallclock secs ( 2.13 usr +  0.00 sys =  2.13 CPU) @ 140.85/s (n=300)
        tr:  0 wallclock secs ( 0.27 usr +  0.00 sys =  0.27 CPU) @ 1111.11/s (n=300)
            (warning: too few iterations for a reliable count)
  while_re:  5 wallclock secs ( 2.11 usr +  0.00 sys =  2.11 CPU) @ 142.18/s (n=300)
 
=== check counts ===
         _while_re  6000
          _m_for_M  6000
        _m_array_M  6000
               _tr  6000
        _m_while_M  6000
          _m_while  6000
          _s_index  6000
            _m_for  6000
          _m_array  6000