DATA is different than STDIN in only one situation as far as I can tell (DATA's starting point isn't at offset zero), but it isn't relevant here.

Your code gives the same undesired behaviour (gigabit 0/1 disable gives success) no matter the source of the input. Junk at the end of the input is never considered an error since you never check that there is no junk at the end of the input.

Other problems:

• Your code allows atm0/1. (Note the lack of space).
• strict and warnings isn't enabled in your grammar.
• Using indirect method calls (such as new Class) is error-prone and best avoided.
• Single-quoted here-docs (<<'WORD') are better than multiline q() constructs since you don't have to escape anything.

Fixes:

use strict;
use warnings;

use Carp              qw( Carp );
use Parse::RecDescent qw( );

$|=1;

my $GlobalConfigMode = <<'__EOI__';
   {
      use strict;
      use warnings;
      my %iftypes = map +($_=>1), qw( fa gigabit atm );
   }

   parse     : Interface /\Z/ { $item[1] }

   Interface : iftype module '/' slot { [ @item[0,1,2,4] ] }

   iftype    : /\w+/ <commit> { $iftypes{$item[1]}||undef } { $item[1]
+ }
             | <error?:invalid iftype>

   module    : /\d+/ { $item[1] }
             | <error:invalid module>

   slot      : /\d+/ { $item[1] }
             | <error:error near slot>
__EOI__

my $ifparser = Parse::RecDescent->new($GlobalConfigMode)
   or croak "Could not create command parser";

while (<STDIN>) {
   chomp;
   if ($ifparser->parse($_)) {
      print "$_ \t success\n" ;
   } else {
      print "$_ Command not found\n";
   }

   print "------------------\n";
}
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