use strict;
use warnings;

use feature qw{ say };

my $str;
my $minY;

$str   =  q{accyyaycayaccyyaaycy};
$minY  = 15;

say $str;

my $yCt   = $str =~ tr{y}{};

my @aPosns;
push @aPosns, pos $str while $str =~ m{(?=a)}g;

my @cPosns;
push @cPosns, pos $str while $str =~ m{(?=c)}g;

while ( ( $yCt < $minY ) && @aPosns )
{
    my $offset = splice @aPosns, rand @aPosns, 1;
    substr $str, $offset, 1, q{y};
    $yCt ++;
}

while ( ( $yCt < $minY ) && @cPosns )
{
    my $offset = splice @cPosns, rand @cPosns, 1;
    substr $str, $offset, 1, q{y};
    $yCt ++;
}

say $str;

##</code><code>##

accyyaycayaccyyaaycy
yycyyyycyyyccyyyyycy

##</code><code>##

accyyaycayaccyyaaycy
accyyaycyyaccyyyaycy