$ perl -le'$_="abc"; s/(.)/$1\x40/sg; print'
a@b@c@

$ perl -le'$_="abc"; s/(?<=.)/\x40/sg; print'
a@b@c@

$ perl -le'$_="abc"; s/(?!^)/\x40/sg; print'
a@b@c@

$ perl -e'
    use Benchmark qw( cmpthese );
    our $i = chr(1) x 1e6;
    cmpthese(-3, {
        cap   => sub { (my $o=our $i) =~ s/(.)/$1\x40/sg;  },
        peek1 => sub { (my $o=our $i) =~ s/(?<=.)/\x40/sg; },
        peek2 => sub { (my $o=our $i) =~ s/(?!^)/\x40/sg;  },
    });
'
        Rate   cap peek1 peek2
cap   1.39/s    --  -25%  -27%
peek1 1.85/s   33%    --   -3%
peek2 1.91/s   37%    4%    --