my @b;
$b[ $_-1 ] = $a[ $_-1 ] - $a[ $_ ] for 1 .. $#a;

##</code><code>##

my @b;
$b[ $_ ] = $a[ $_ ] - $a[ $_+1 ] for 0 .. $#a-1;

##</code><code>##

my @b = @a;
$b[ $_ ] -= $b[ $_+1 ] for 0 .. $#b-1;
pop @b;