use strict;
use warnings;

my $string = "aabbcc";

if($string =~ m{^(aabb)(.*)$})
{
   $string = substr($1,0,length($1)-1) . "X" . $2;
}

print $string;
[download]

or to be more kloogy..

print $string = substr($1,0,length($1)-1) . "X" . $2 if $string =~ m{^
+(aabb)(.*)$};
[download]