Take the following code, for instance:

my @a = (1, 2, 3);

add(@a);

sub add {
  for (@_) {
    $_++;
  }
}
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Now try printing @a; it contains (2, 3, 4), because the add() function acted directly upon it's arguments.

Likewise, your statements are changing the first input parameter directly, by performing substitutions upon $_[0], which is the first value of @_.