Import pragmas like strict and warnings into callers lexical scope

youwin has asked for the wisdom of the Perl Monks concerning the following question:

Oh wise and omniscient monks,

Im trying to understand why this works to import strict into the callers package:

# Foo.pm
package Foo;
sub import {
    strict->import;
}

# script
use Foo;
$yup = 5; # bang! (Global symbol "$yup" requires explicit...)
[download]

I know that normally to import package based modules, you can write something like this:

# Foo.pm
package Foo;
sub import {
    my $caller = caller;
    eval "package $caller; use Data::Dumper;";
}

# script
use Foo;
print Dumper(\@ARGV); # $VAR1 = ...
[download]

Which makes sense because the use statement thinks it in the callers package. strict is strange because it imports into the lexical scope of the caller, and I thought that would be the block of the import routine in the first example above. So how does it know to enable strict for the callers lexical scope?

Comment on Import pragmas like strict and warnings into callers lexical scope Select or Download Code

Replies are listed 'Best First'.
Re: Import pragmas like strict and warnings into callers lexical scope by ikegami (Patriarch) on Feb 11, 2011 at 19:44 UTC
All `strict->import` (normally called via `use strict`) does is modify global var `$^H`. The compiler uses `$^H` to determine if it should throw strict errors, among other things. The compiler localises `$^H` to the block being compiled. This is what produces the lexical scoping of the pragma. `{ # $^H is localised by the compiler # when it compiles this. use strict; # Changes $^H ... } # $^H is restored by the compiler # when it compiles this, # undoing changes in strictures.` [download] In your example, the block being compiled when `strict->import` is called is your script's file block, so the strictures stay in effect until the end of the file. Similarly scoped hash `%^H` is available to modules that want to be lexical pragmas without having to fight Perl for the limited bits in `$^H`. Update: Added code example. Added mention of `%^H`.	[reply] [d/l] [select]
Re^2: Import pragmas like strict and warnings into callers lexical scope by youwin (Beadle) on Feb 11, 2011 at 22:35 UTC
Is it special that Perl restores $^H at the end of the enclosing block? Like could I write an import statement that does the same with a different global variable ($main::G for instance)?	[reply]
Re^3: Import pragmas like strict and warnings into callers lexical scope by ikegami (Patriarch) on Feb 11, 2011 at 23:12 UTC
It's special to $^H and %^H, but %^H was created to give you access to that behaviour `$^H{'mypragma'} = ...;` [download] `$^H{'mypragma::G'} = ...;` [download]	[reply] [d/l] [select]