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...)
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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 = ...
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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?

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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]


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