There are two problems with the code...

First, it's not case-insensitive...

Secondly, an answer such as "y should I?" will match :-)

This will do the trick:

print "Are you sure? [y]";
chomp(my $ans = <STDIN>);
$ans ||= "y";
if ($ans =~ m!^y(es)?$!i) {
   # do something
} else {
   # do otherthing
}
[download]

And another approach:

#!/usr/bin/perl
print "Are you sure? [y]";
if (<> =~ /^(y(es)?)?\n$/i) {
  # answer was yes
}
else {
  # answer was no
}
[download]