在Perl中，bareword filehandle包变量是否可变？

Question

The legacy bareword filehandle looks in global scope, is it parsed as package variable? Why isn't it prepended with punctuation sign ( $ )?

open FILE, 'file.txt';

Answer 1

If you look at perldoc -f ref you will see that there are more data types than there are identifying sigils. The only defined ones are

• $ - SCALAR
• @ - ARRAY
• % - HASH
• & - CODE

the rest are identified by context.

If you write

package Pack;

open FH, '<', 'file';

then FH appears in the Pack symbol table as the IO variable Pack::FH . It isn't preceded by a dollar $ because it is an IO variable, not a SCALAR . Without a preceding package statement, it will be placed in the default main namespace. So with your example

open FILE, 'file.txt'

you can then read from the FILE handle using the fully-qualified main::FILE like this

while (<main::FILE>) {
  :
  :
}

Answer 2

Used to be a convention in Perl - filehandles were all uppercase, and globally scoped.

This started with STDIN (and the other standard file streams) and so you could do:

while ( <STDIN> ) {
    print;
}

Perl's open command supports a 'legacy' mode of opening, which is:

open ( FILEHANDLE, ">filename.txt" );

It's not recommended to use this any more, but it can't be removed without breaking legacy code.

Instead you should:

 open ( my $filehandle, ">", 'filename.txt' ) or die $!;
 print {$filehandle} "Some Text\n";

This has two advantages:

• your filehandle is lexically scoped, so you don't have to worry about a module writing to 'OUTPUT' or similar.
• Because it's lexically scoped, it automatically closes when it goes out of scope.
• 3 argument open avoids injection exploits.