Good point. I'll see if I can set up some benchmarking and post the results.

Update:

Running the code below under ubuntu bash under windows with LC_NUMERIC set to de_DE.iso88591 indicates that s///r is about 20% faster than y//r. Both are in the order of millions per second, though.

And as yet another update, the order does switch between runs so it's not a consistent outcome.

Results:

de_DE.iso88591
3,1
3.1
3.1
                Rate y_sprintf s_sprintf   sprintf
y_sprintf  4646523/s        --      -19%      -63%
s_sprintf  5721423/s       23%        --      -54%
sprintf   12396265/s      167%      117%        --
[download]

Code:

use Benchmark qw {:all};
use 5.016;

local $| = 1;

use POSIX qw /locale_h/;
use locale;
print POSIX::setlocale(LC_NUMERIC) . "\n";

my $x;
$x = sprintf "%.2g", 3.14; 
print "$x\n";
$x = sprintf ("%.2g", 3.14) =~ y/,/./r; 
print "$x\n";
$x = sprintf ("%.2g", 3.14) =~ s/,/./r; 
print "$x\n";


cmpthese (
    -3,
    {
        sprintf   => 'use strict; use warnings; my $x = sprintf ("%.2g
+", 3.14)',
        y_sprintf => 'use strict; use warnings; my $x = sprintf ("%.2g
+", 3.14) =~ y/,/./r',
        s_sprintf => 'use strict; use warnings; my $x = sprintf ("%.2g
+", 3.14) =~ s/,/./r',
    }
);
[download]

(post was further updated to distinguish results from code)