如何使用仅在运行时才知道的Perl软件包？

eval "require $ENV{a}";

“use”在这里不起作用,因为它只在eval的上下文中导入.

正如@Manni所说,其实最好是使用require.引用人perlfunc：

If EXPR is a bareword,the require assumes a ".pm" extension and 
replaces "::" with "/" in the filename for you,to make it easy to 
load standard modules.  This form of  loading of modules does not 
risk altering your namespace.

In other words,if you try this:

        require Foo::Bar;    # a splendid bareword

The require function will actually look for the "Foo/Bar.pm" file 
in the directories specified in the @INC array.

But if you try this:

        $class = 'Foo::Bar';
        require $class;      # $class is not a bareword
    #or
        require "Foo::Bar";  # not a bareword because of the ""

The require function will look for the "Foo::Bar" file in the @INC 
array and will complain about not finding "Foo::Bar" there.  In this 
case you can do:

        eval "require $class";