Perl Language Subroutines Subroutine arguments are passed by reference (except those in signatures)
Example
Subroutine arguments in Perl are passed by reference, unless they are in the signature. This means that the members of the @_ array inside the sub are just aliases to the actual arguments. In the following example, $text in the main program is left modified after the subroutine call because $_[0] inside the sub is actually just a different name for the same variable. The second invocation throws an error because a string literal is not a variable and therefore can't be modified.
use feature 'say';
sub edit {
$_[0] =~ s/world/sub/;
}
my $text = "Hello, world!";
edit($text);
say $text; # Hello, sub!
edit("Hello, world!"); # Error: Modification of a read-only value attempted
To avoid clobbering your caller's variables it is therefore important to copy @_ to locally scoped variables (my ...) as described under "Creating subroutines".
