How about finding the positions of the 'a' and 'c' characters using a regex look-ahead in a global match along with pos and then randomly replacing 'a's at those positions until there are none left before, if necessary, going on to the 'c's.

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;
[download]

This produces

accyyaycayaccyyaaycy
yycyyyycyyyccyyyyycy
[download]

Running this again but with $minY reduced to 10 gives this

accyyaycayaccyyaaycy
accyyaycyyaccyyyaycy
[download]

I hope this is helpful.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.