WRT this recent discussion thread: Forget map, if you want to take it up just that extra quantum energy level that allows you to achieve truly epic abuse, use grep:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my @ra = (1, 2, 3, 4);
 ;;
 my @rb;
 grep $rb[ $_-1 ] = $ra[ $_-1 ] + $ra[ $_ ], 1 .. $#ra;
 dd \@rb;
"
[3, 5, 7]
[download]

Update: Also works with
    grep $rb[ $_ ] = $ra[ $_ ] + $ra[ $_+1 ], 0 .. $#ra-1;
with one fewer arithmetic operation per iteration!