use strict;
use warnings;

my $text    = q{abaacaa};
my $repCt   = 3;
my $replace = q{a};
my $len     = length $replace;
my $with    = q{aa};

print qq{$text\n};

my @posns;
push @posns, pos $text while $text =~ m{(?=$replace)}g;
print qq{@posns\n};

substr $text, $_, $len, $with
   for reverse @posns[ 0 .. $repCt - 1 ];

print qq{$text\n};

##</code><code>##

abaacaa
0 2 3 5 6
aabaaaacaa