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

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

substr((  substr( $p, 0, 1 ) = '123' ), 0, 1 ) = 'x';
substr((( substr  $q, 0, 1 ) = '123' ), 0, 1 ) = 'x';

say $p;
say $q;