perl - Which one is good practice, a lexical filehandle or a typeglob?

Question

Some say we should use a lexical filehandle instead of a typeglob, like this:

open $fh, $filename;

But most Perl books, including The Llama Book, use a typeglob, like this:

open LOGFILE, $filename;

So what are the differences? Which one is considered a better practice?

Answer 1

The earliest edition of the Llama Book is from 1993, before lexical filehandles were part of the Perl language. Lexical filehandles are a better practice for a variety of reasons. The most important disadvantages of typeglobs are

1. they are always global in scope, which can lead to insidious bugs like this one:

   sub doSomething {
     my ($input) = @_;
     # let's compare $input to something we read from another file
     open(F, "<", $anotherFile);
     @F = <F>; 
     close F;
     do_some_comparison($input, @F);
   }
   
   open(F, "<", $myfile);
   while (<F>) {
       doSomething($_);   # do'h -- just closed the F filehandle
   }
   close F;
   

2. they are harder to pass to a subroutine than a lexical filehandle

   package package1;
   sub log_time { # print timestamp to filehandle
       my ($fh) = @_;
       print $fh scalar localtime, "
   ";
   }
   
   package package2;
   open GLOB, '>', 'log1';
   open $lexical, '>', 'log2';
   
   package1::log_time($lexical);         # works as expected
   package1::log_time(GLOB);             # doesn't work
   package1::log_time('GLOB');           # doesn't work
   package1::log_time(*GLOB);            # works
   package1::log_time(package2::GLOB);   # works
   package1::log_time('package2::GLOB'); # works
   

See also: Why is three-argument open calls with autovivified filehandles a Perl best practice?