# Assume you have
use Compress::Zlib;
use Carp;

# Here is the function.  It takes text, and returns 3
# possible answers.  1 if the text looks unencrypted.
# 0 if it looks encrypted.  And undef if it is not able
# to tell reliably.
sub text_is_normal {
  my $text = shift;
  length($text) < 50
    ? undef
    : (length(compress($text)) < 0.8*length($text));
}