Umm, neither of these calls a subroutine. splice is a list operator, not a sub -- no call stack need be generated, because no perl code is being called.

#!perl -w
use strict;
use Benchmark;

my @ary = 0 .. 100;

sub test {
 $a++;
}

Benchmark::cmpthese ( -1, {
    direct => sub {$a++},
    sub    => sub {test}
   }
);
__END__
Benchmark: running direct, sub, each for at least 1 CPU seconds...
    direct:  1 wallclock secs ( 1.48 usr +  0.00 sys =  1.48 CPU) @ 34
+6460.81/s (n=512762)
       sub:  1 wallclock secs ( 1.00 usr +  0.00 sys =  1.00 CPU) @ 13
+7843.00/s (n=137843)
           Rate    sub direct
sub    137843/s     --   -60%
direct 346461/s   151%     --
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