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

##</code><code>##

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