One might argue that Perl always does Pass-By-Reference, but protects us from ourselves.

@_ holds the arguments passed to a subroutine, and it is common idiom to see something like:

sub mySub{
  my $Arg = shift;
}

sub mySub2{
  my ($Arg) = @_;
}
[download]

Why is that? Why don't we just use the @_ array directly?

First, there is the laudable goal of more readable code, which is sufficient reason, in itself, to rename variables away from cryptic things like $_[3]. But really, we copy values out of @_ because (from the man page) "its elements are aliases for the actual scalar parameters."

In short, this means that if you modify an element of @_ in your subroutine, you are also modifying the original argument. This is almost never the expected behavior! Further, if the argument is not updatable (like a literal value, or a constant), your program will die with an error like "Modification of a read-only value attempted."

Consider:

sub test{
  $_[0] = 'New Value';
}

my $Var = 'Hi there';
print "$Var\n";
test ($Var);
print "$Var\n";
[download]

will print out:

Hi there
New Value
[download]

So, yes, you can do pass-by-reference in Perl, even without backslashes; but it is almost always better (some would leave out the "almost" in this statement) to make your caller explicitly pass you a reference if you intend to modify a value.

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