Your code only checks for "yes". If the user entered "Sure!", you would get the "Hmm - that stinks. You need a blue rock." answer. Better test for "no" too:

$|++; # make sure we turn off buffering.
print "Is the rock blue? yes or no: ";
my $response = <STDIN>;
chomp $response; # get rid of the enter key
if ($response eq 'yes')
{
  print "Great, it's blue!\n";
}
elsif ($response eq 'no')
{
  print "Hmm - that stinks.  You need a blue rock.\n";
}
else
{
  print "yes or no, boy!\n";
}
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