my $t = 0;
grep { $t^=1 } LIST
[download]

using the XOR operator in XOR-equal is bordering on line-noise. This seems to have a low likely hood of being understood on the first pass by the next person to see it. (This next person had to go check perlop). That said, I'm totally using it the next time I play golf.

Mixing both of your two forms we get these nice options:

my $t = 0; #even elements
#my $t= 1; #odd elements
grep { $t++ % 2 } LIST
[download]

my $t = 0;
map { $t++ %2 ? $_ : () } LIST
[download]

Am I wrong to expect people to be more familiar with '%' than with '^'?