"Passing by reference" ( reference in computer science lingo) is the default in Perl since $_[0] is an alias . (NB: "alias" != "reference" in Perl)

For instance you can do

 

my $string = "This is a string";

printWthStrRef($string);    # passing string by alias


sub printWthStrRef {
    my $ref = \( $_[0] );

    print $$ref;       # prints the string
    print $_[0];       # prints the string
    $_[0] = "new";     # overwrite original $string

}
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You only need explicit references when passing multiple complex variables like arrays or hashes, to avoid flattening to lists.

(And even here you could use prototypes, but this is too far fetched here)

( disclaimer: I wrote this hours ago but the website was unresponsive for me)