Consider

sub foo {
  my $x=shift;
  $x="Changed1";
  $_[0]="Changed2";
}
my $i="Orig1";
my $j="Orig2";
print "$i,$j\n";
foo($i,$j);
print "$i,$j\n";
__END__
Orig1,Orig2
Orig1,Changed2
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Assigning to $_[$x] is the same as assigning to the original value that was passed into the subroutine. This is because its an alias. Its not a copy. IOW, its where two or more variable names refer to the same memory location. Its much as if you did

sub bar { my $ref=shift; $$ref='changed' }
bar(\$j);
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Except you dont have to deref before you do the assignment.

for() aliases, as does map, and a few other places where it can be useful to have this behaviour.