use strict;
use warnings;

my $needed_ys = 7;
my $str       = 'Why do you need 7 ys when cll you hve re as and cs';
my $orig      = $str;
my @stash     = qw(a c);

my @existing = $str =~ /y/g;
my $missing = $needed_ys - @existing;

foreach my $letter (@stash) {

    while ( $missing > 0 ) {
       last unless $str =~ s/$letter/y/;
       $missing -= 1;
    }
}

print 'Original: ', $orig, "\n";
print 'Result:   ', $str, "\n";