Re: Cost of list assignment -

[BrowserUk]

but wondering if this costs more than than doing it otherways

Yes, measurably so:

@a=1..1e6; 
cmpthese -3,{
    a => q[ @a[ $_-1, $_ ] = @a[ $_, $_-1 ] for 1 .. $#a; ],
    b => q[ my$t; $t = $a[ $_-1 ], $a[ $_-1 ] = $a[ $_ ], $a[ $_ ] = $
+t for 1 .. $#a; ]
};;
    Rate   a   b
a 1.58/s  -- -5%
b 1.66/s  5%  --

but the difference is tiny, so only becomes significant in inner loops with high iteration factors.

Other times the clarity is worth the cost.