Careful there: only the second number is optional. If you miss out the first number perl will fail to recognise the braces as a quantifier, and so it gets treated as a literal string to match instead:

  zen% perl -wle 'print "miss" if "aa" !~ /^a{,3}$/'
  miss
  zen% perl -wle 'print "match" if "aa" =~ /^a{0,3}$/'
  match
  zen% perl -wle 'print "match" if "a{,3}" =~ /^a{,3}$/'
  match
  zen%
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