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substr $string, 0, 30; 
  # will extract UP TO the first 30 characters of the string.

...

length($string) == 30;
  # tests if the string is 30 characters long.

my @strings = (
  join ('', 'a' .. 'z' ),
  join ('', 'a' .. 'z', 0 .. 3 ),
...
  print 'regex  : ', ( $string =~ m/.{30}/ ), "\n";
  print 'regex  : ', ( $string =~ m/(.{30})/ ), "\n";
}

string : abcdefghijklmnopqrstuvwxyz
length : 26
substr : abcdefghijklmnopqrstuvwxyz
...
substr : abcdefghijklmnopqrstuvwxyz0123
regex  : 1
regex  : abcdefghijklmnopqrstuvwxyz0123