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my $x = "def";
say ref( \substr( $x, 0, 0 ) );  # LVALUE
substr( $x, 0, 0 ) = "abc";
say $x;                          # abcdef

my $x = "def";
my $r = \substr( $x, 0, 0 );
say ref( $r );                   # LVALUE
$$r = "abc";
say $x;                          # abcdef

$ perl -Mv5.014 -e'
   sub f { my $REF = \$_[0]; say ref( $REF ); }
   f substr( "", 0, 0 );
'
LVALUE

$ perl -Mv5.014 -e'
   sub f { say ref( \$_[0] ); }
   f substr( "", 0, 0 );
'
LVALUE