use strict;
use warnings;

my $window = 30.0 / 1440; # 30 minutes
my $error_file = 'error.txt';
my $error_message = shift; 
chomp($error_message);
if (! -e $error_file) {
  # generate warning and then
  write_error_file();
} else {
  chomp(my $old_error = `cat $error_file`);
  # two conditions warrant a warning 
  # a) either a new type of error or b) it has been 
  # more than 30 minutes since we last saw this one
  if (($old_error ne $error_message) || (-M $error_file > $window)) {
    # generate warning and then
    write_error_file();
  }
}

sub write_error_file {
  my $ofh;
  print "New error $error_message\n";
  open $ofh,'>',$error_file; 
  print $ofh $error_message;
  close $ofh;
}