The implict reference is not a hard reference, as perlref speaks of, but an aliasing that occurs. For example:

sub x {
  $_[0] = "in x";
}
my $foo = "outside";
x($foo);
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... will set $foo to "in x". The thing in @_ has become an alias for $foo. However, you won't get this behavior if you explictly copy the value out of @_, as in:

sub x {
  my($foo) = @_;
  $foo = "in x";
}
my $foo = "outside";
x($foo);
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When you copy the value, you are not working with an alias to the thing passed in, but you a working with a copy, bringing you the pass-by-value semantics that are probably more intutitive to you.