What is the difference between <stdin> and <STDIN>?

问题

When I use <stdin> in Perl module (*.pm) files it's not reading input from the keyboard, but when I use <STDIN> in the same place it works fine.

Why is it not getting input when I use <stdin>?

回答1:

STDIN is the documented filehandle. There exists stdin as well, which is aliased to STDIN, but it only works in the main:: package: main::stdin is the same as main::STDIN (as documented in perlop - Perl operators and precedence).

In a package, therefore,

package My::Package;
sub xx {
    print while <stdin>;
}

stdin is interpreted as My::Package::stdin, which doesn't exist. You can use main::stdin from a package, but using the standard STDIN (which always points to main::STDIN, even from a package) is much cleaner.

回答2:

Didn't know about this, but found it documented in a throw-away paragraph in perlop

The filehandles STDIN, STDOUT, and STDERR are predefined. (The filehandles stdin, stdout, and stderr will also work except in packages, where they would be interpreted as local identifiers rather than global.) Additional filehandles may be created with the open() function, amongst others. See perlopentut and "open" in perlfunc for details on this.

来源：https://stackoverflow.com/questions/31185416/what-is-the-difference-between-stdin-and-stdin