As davido points out, my question is : how to get rid of the warning.

And in as few keystrokes as possible, without a significant time overhead.

Finally, I decided to try the obvious solution : turn off warnings locally.

sub split_simple_quiet
{
    no warnings;
    my $n = scalar split ',', $str;
    #print "$n\n";
}
[download]

This does the job and costs almost nothing in time, as the expanded benchmark shows:

Use of implicit split to @_ is deprecated at H:\dev\perl\perlmonks\ben
+ch.pl line 23.
Benchmark: timing 10000 iterations of split_anonymous, split_simple, s
+plit_simple_quiet...
split_anonymous: 24 wallclock secs (15.55 usr +  0.00 sys = 15.55 CPU)
+ @ 643.00/s (n=10000)
split_simple: 20 wallclock secs (12.23 usr +  0.00 sys = 12.23 CPU) @ 
+817.80/s (n=10000)
split_simple_quiet: 18 wallclock secs (12.15 usr +  0.01 sys = 12.16 C
+PU) @ 822.57/s (n=10000)
                    Rate   split_anonymous       split_simple split_si
+mple_quiet
split_anonymous    643/s                --               -21%         
+      -22%
split_simple       818/s               27%                 --         
+       -1%
split_simple_quiet 823/s               28%                 1%         
+        --
[download]

Rudif