*Only* scalars can be passed to a sub as parameters, and they can *only* be passed by reference. If you modify @_, the actual variable passed in gets modified! @_ is really a list of aliases to the argument list.

Try it: 

sub foo { $_[0] = 17 }
my $x=0;
foo $x;
print "$x\n"

[download]
And foo @x does what you'd expect given the aliasing.

Of course, the usual course of action is to copy them into my variables. Which is why it's perceived as call by value.

In reply to Re: Is it possible to do pass by reference in Perl? by ariels
in thread Is it possible to do pass by reference in Perl? by Anonymous Monk

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