You can explicitly pass a reference if you like, or you can use the dark voodoo of prototypes. You could define

sub test(\$) {
    my ($scalar_ref) = @_;
    $$scalar_ref =~ tr/a-m/N-Z/;
    substr($$scalar_ref, 0, 0) = "prepended sumthin'";
}
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and it should do what you want. Just make calls like

until your heart is content. You just have to make sure that test(\$) is seen before making the call. And be careful what you pass as an argument to test() because prototypes can work differently than you think they should.