sub get_num_lines_method1 {
   my ($file_name) = @_;

   my $num_lines = 0;
   local *FILE;
   open(FILE, $file_name);
   $num_lines++ while (<FILE>);
   close(FILE);
   return $num_lines;
}

sub get_num_lines_method2 {
   # Really same as get_num_lines_method1.
   my ($file_name) = @_;

   my $num_lines = 0;
   local *FILE;
   open(FILE, $file_name);
   while (<FILE>)
   {
      $num_lines++;
   }
   close(FILE);
   return $num_lines;
}

sub get_num_lines_method3 {
   # Loads the whole file into an array.
   my ($file_name) = @_;

   local *FILE;
   open(FILE, $file_name);
   my @file = <FILE>;
   close(FILE);
   return scalar(@file);
}

sub get_num_lines_method4 {
   # Loads the whole file into a scalar.
   my ($file_name) = @_;

   local *FILE;
   open(FILE, $file_name);
   local $/;
   my $file = <FILE>;
   close(FILE);
   return $file =~ tr/\n/\n/;
}

printf("Method 1: %d\n", get_num_lines_method1($0));
printf("Method 2: %d\n", get_num_lines_method2($0));
printf("Method 3: %d\n", get_num_lines_method3($0));
printf("Method 4: %d\n", get_num_lines_method4($0));

__END__
output:
=======
Method 1: 62
Method 2: 62
Method 3: 62
Method 4: 62