...
$aod[$yb] = rankdwn( $aod[$yb], $howManyYsDoWeWant );
...

sub rankdwn {
    my ( $str, $incrsdel ) = @_;

    my $cnd_y = $str =~ tr/y/y/;

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

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

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

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

    return $str;
}