join ', ', @a

##</code><code>##

my $string = join ', ', @a[$[..$#a-1];
$string .= ", and $a[-1]";

# or in one line
my $str = join ', ', @a[$[..$#a-1], "and $a[-1]";

##</code><code>##

my $str = @a > 1 ? join ', ', @a[$[..$#a-1], "and $a[-1]" : $a[0];