Works with any number of elements, non-destructive:

sub a { @_<2 ? @_ : (pop,shift,&b) }
sub b { @_<2 ? @_ : (shift,pop,&a) }
print a(1..8), "\n";
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Works with a multiple of 4 elements, non-destructive:

sub a { @_ ? (@_[-1,0,1,-2],a(@_[2..$#_-2])) : () }
print a(1..8), "\n";
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Update: Even shorter,

Works with a multiple of 4 elements, non-destructive:

sub a { @_ ? (pop,shift,shift,pop,&a) : () }
print a(1..8), "\n";
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