knexus,
It would be a good way to handle the problem, but I don't think you are doing it quite right using conventional syntax. 
my $large_scalar = 'a huge string like 64K or something';

modify_largescalar(\$large_scalar);

sub modify_largescalar {
    my $scalar_ref = shift;
    # Code to modify $$scalar_ref;
}

[download]
Your use of prototypes isn't helping (but maybe you need that for some other reason). The thing is you are not creating refs until you are inside the sub - which means you are already copying the large scalar. You are using the fact that the elements of @_ are aliases to the variables and avoiding copying by assigning new variable names to references to the aliases. I do not know if this will avoid copying, but it certainly isn't conventional syntax. I also wouldn't worry about optimization of these large scalars unless they are truly large and/or you are calling these subs many times in a tight loop.

Hope this helps - L~R

Updated after CombatSquirrel pointed something out I took another look at perldoc perlsub and conferred with belg4mit in the CB.

In reply to Re: Memory Use and/or efficiency passing scalars to subs by Limbic~Region
in thread Memory Use and/or efficiency passing scalars to subs by knexus

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