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use strict;
use warnings;
...
Dump $r;                    # ref target is now POK, PV is "Z"
say ${ thaw freeze $r };    # "Z"

use strict;
use warnings;
...
                            # full of magic, above.
say ${ thaw freeze \foo };  # 42

@result = mce_map { ...work with $_ } substr(...)
@result = mce_map { ...work with $$_ } \substr(...)
@result = mce_map { ...work with $$_ } \( my $s = substr(...))