I would really drop the our and typeglob usage here. I feel like it's "overkill" for what [id://tomazos] is trying to do.

The following is also an interesting solution, that is quite similar to [id://friedo]'s one, but uses prototype to move referencing backslash from main script into the function code.

#!/usr/bin/perl
use strict;
use warnings;

sub modify (\$) {
    my $string_ref = $_[0];
    $$string_ref =~ s/a/x/g;
}

my $string = 'abc';

print "before `$string'\n"; # abc
modify $string;
print "after  `$string'\n"; # xbc
[download]

Anyway, probably it is best to leave the \ char inside the main script. In this way, it is immediately clear that you're passing a reference, and not the real scalar. In the latter case, you could be tempted to think that a return value is also expected from the modify() function, which actually is not.