it will split into 3 parts

I'm not sure there's any guarantee perl will do that optimization for you. If you want three parts, just tell it so:

$foo = (split/:/,$_,3)[1];
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Update: You seem to be right about the optimization. The following benchmark shows nearly identical times:

cchan.acsys.com.1.256% cat aaa3.pl 
use Benchmark; 
my $count = 1000000;
our $str = "a:b:c:d:e:ff:g:h:i:j:k:l:m:n:o:p:q:r:s:t:u:v:w:x:y:z";
timethese($count, {
          'slice' => sub { (undef, my $foo) = split /:/,$str; },
               'count' => sub { my $foo = (split/:/,$str,3)[1]; },
           });
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Benchmark: timing 1000000 iterations of count, slice...
     count:  2 wallclock secs ( 3.18 usr +  0.01 sys =  3.19 CPU) @ 31
+3479.62/s (n=1000000)
     slice:  3 wallclock secs ( 3.04 usr +  0.06 sys =  3.10 CPU) @ 32
+2580.65/s (n=1000000)
[download]

However, giving the split limit explicitly is clearer IMHO.