# a recursive approach, similar to BrowserUK's
sub swap1 
{
    my @t = ( pop, pop );
    ( @_ ? &swap1 : (), @t )
}


# an iterative approach
sub swap2
{
    my @t;
    unshift @t, ( pop, pop ) while @_;
    @t
}
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I've tried to avoid any reference to the index whatsoever, and also reverse, which I think is too "obvious".
BrowserUK's solution is superior in that it avoids temporary variables as well.