If the expected input is y or yes in a line then following code can be used simply.

while(<>)  {
        print "yes\n" if /(^y$|^yes$)/i;
}
[download]

So could

while(<>)  {
        print "yes\n" if /^y$/i || /^yes$/i;
}
[download]

while(<>)  {
        print "yes\n" if /^y$/i;
        print "yes\n" if /^yes$/i;
}
[download]

By the way, useless capturing greatly slows down pattern matching.