#!/usr/bin/perl -w
use strict;

my $string = 'John is following jane';

# Specific 
$string =~ s/[jJ]ane/##Jane##/;

# To preserve j or J in replacement
# $string =~ s/([jJ]ane)/##$1##/;

# More generic
# $string =~ s/(John is following) (\w+)/$1 \u$2/;