Another way would be to combine a regular expression look-ahead with pos to find all of the potential positions in the text where a substitution might be made and then use substr to make the substitutions, working from the last substitution forwards to avoid the positions going out of kilter.

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};
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The output.

abaacaa
0 2 3 5 6
aabaaaacaa
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I hope this is of interest.