use strict;
use warnings;
use feature 'say';

my $p = 'abc';
my $q = 'abc';

my $ref_p = \scalar(   substr( $p, 0, 1 )   = '123' );
my $ref_q = \scalar( ( substr( $q, 0, 1 ) ) = '123' );

say $$ref_p;  # 123 
say $$ref_q;  # 1

substr( $$ref_p, 0, 1 ) = 'x';
substr( $$ref_q, 0, 1 ) = 'x';

say $$ref_p;  # x23
say $$ref_q;  # x

say $p;       # x23bc
say $q;       # 123bc