die "..." 
      if $string =~ /:|.{17}/;
[download]

This accepts all strings less than 17 characters. perl514 wants only strings exactly 16 characters.

>perl -wMstrict -le
"my $string = 'foo';
 ;;
 die '...' if $string =~ /:|.{17}/;
 ;;
 print qq{processing '$string'};
"
processing 'foo'
[download]