It shouldn't be @_[0] because that's the arrayslice consisting of the 0th element. If you turn warnings on, it will tell you about this. Arrayslicing is really cool. Try the following snippet:

my @x = 0 .. 9;
print "@x[2..4]\n";
print "@x[3,5,7]\n";
my @y = 4..6;
print "@x[@y]\n";
@x[7..9] = @y;
print "@x\n";
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The reason why you don't want to use the arrayslice form when you want to extract a single element is, while it has been special-cased to work in the case of my $x = @x[2]; (it should assign 1 and not 2), it won't work in this case:

sub foo {
    wantarray ? 3 : 5;
}

my @x;
@x[2] = foo();
my $x = foo();
print "$x[2] <-> $x\n";
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@x[2] is list context, not scalar context.