#!/usr/bin/perl
use strict;
use warnings;
use feature 'say';

sub replaceChar {
    my ($str, $char, $replace) = @_;
    substr($str, index($str, $char), length($char), $replace);
    return $str;
}

my $s = "(a+b)*c";
my $leftParenthesis = "(";
my $rightParenthesis = ")";
my $leftCurlyBracket = "[";
my $rightCurlyBracket = "]";

my $str = replaceChar($s, $leftParenthesis, $leftCurlyBracket);
say $str;
say replaceChar($str, $rightParenthesis, $rightCurlyBracket);

__END__

$ perl test.pl
[a+b)*c
[a+b]*c