#!/usr/bin/perl
use v5.14;

my $string = '0123456789';
my $index  = 1;             # second character
my $replacement = 'x';

substr( 
    $string,
    $index,         # starting from 0
    1,              # how many characters
    $replacement,
);

say $string;        # will print out '0x23456789' as expected