Using index and substr seems to be quicker still.

use strict;
use warnings;

use Benchmark q{cmpthese};

my $rsIndex = sub
   {
       my $str = q{MYKEY=MYVAL=2};
       my $pos = index $str, q{=};
       my $key = substr $str, 0, $pos;
       my $val = substr $str, $pos + 1;
       return [$key, $val];
   };

my $rsMatch = sub
   {
       my ($key, $val) = q{MYKEY=MYVAL=2} =~ /^([^=]*)=(.*)/s;
       return [$key, $val];
   };

my $rsSplit = sub
   {
       my ($key, $val) = split m{=(?=.*=)}, q{MYKEY=MYVAL=2};
       return [$key, $val];
   };

cmpthese(-5,
   {
      Index => $rsIndex,
      Match => $rsMatch,
      Split => $rsSplit
   });
[download]

         Rate Split Match Index
Split 26498/s    --   -6%  -33%
Match 28068/s    6%    --  -29%
Index 39627/s   50%   41%    --
[download]

Cheers,

JohnGG