Perl v5.26x attributes like decorators/overlays. Subs pre and post processing

Question

I'm trying to understand Perl attributes for subs. For example, I expected to got as output:

before
middle
after
125

But, I have:

before
middle
after
middle
124

How to use attributes as overlays for main sub?

use Attribute::Handlers ;
use Data::Dumper ;

sub decorator( $\*&\@\@$$$ ):ATTR {
    my ( $package , $symbol, $referent, $attr , $data, $phase, $filename, $linenum ) = @_ ;
    my ( $before , $after ) = @$data ;

    # before processing
    $before->( @_ ) ;

    # decorated sub
    my $result = $referent->( ) ;

    # after processing
    $after->( $result )
}

sub c( $ ):decorator(
    # before
    sub( $\*&\@\@$$$ ) {
        warn 'before' ;
    } ,
    # after
    sub( $ ) {
        warn 'after' ;

        shift( @_ ) + 1
    }
) {
    warn 'middle' ;

    splice( @_ , 1 ) + 1
}

print( __PACKAGE__->c( 123 ) ) ;

For admins. Please, stop moking!

Answer 1

The key piece of information you're missing is that decorator is called once at compile time.

decorator will need to replace &c with a sub wrapping the true &c to get the desired result.

use Attribute::Handlers;
use Sub::Name qw( subname );

sub Wrap :ATTR(CODE) {
   my ( $pkg, $sym, $old_sub, $attr, $data, $phase, $fn, $ln ) = @_;
   my ( $before, $after ) = @$data;

   my $name = *{ $sym }{ NAME };
 
   my $new_sub = subname "wrapped_$name", sub {
      wantarray
      ? $after->(         $old_sub->( $before->( @_ ) )   )
      : $after->( scalar( $old_sub->( $before->( @_ ) ) ) )
   };

   no warnings qw( redefine );
   *$sym = $new_sub;
}

Tested:

sub f :Wrap(
    sub { $_[0], $_[1]+50 },
    sub { $_[0]+7000 },
) {
    $_[1]+600
}

say __PACKAGE__->f( 4 );  # 7654

Note: This uses the calling convention from the OP's own answer, which differs from the calling convention in the the OP's question.