join ', ', @a
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Update: I just noticed the extra 'and' for the last thing, so:

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

# or in one line
my $str = join ', ', @a[$[..$#a-1], "and $a[-1]";
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BlaisePascal makes the good point that this assumes a list of size greater then 1. This too can be solved easily:

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