I have a few little improvements.

• strict and warnings are specified for your program, but not for the code snippets in your grammar.
• There is an undue requirement on the user (i.e. the calling script) to provide concat.
• $decommendedText should not be global, and the user (i.e. the calling script) should not have to initialize it.
• There is no check for end-of-file. If the parser fails halfway through, there's no way of knowing.

The following addresses these issues.

use strict;
use warnings;
use Parse::RecDescent;

my $decomment = <<'GRAMMAR';

{
   use strict;
   use warnings;

   sub concat { $decommendedText .= $_[0]; }
}

file    : <rulevar: local $decommendedText = ''>
        | block(s) /\Z/
            {$return = $decommendedText; 1;}

block   : string
            {concat ($item{string}); 1;}
        | m{((?!/\*|"|').)+}s
            {concat ($item[-1]); 1;}
        | comment
            {concat ($item{comment}); 1;}
            
string  : /"([^"]|\\")*"/
            {$return = $item[-1] . ($text =~ /^\n/ ? "\n" : ''); 1;}
        | /'([^']|\\')*'/
            {$return = $item[-1] . ($text =~ /^\n/ ? "\n" : ''); 1;}
        
comment : '/*' commentBlock '*/'
            {$return = $text =~ /^\n/ ? "\n" : ''; 1;}

commentBlock   :  m{((?! \*/ | /\* ).)*}sx comment m{((?! \*/ | /\* ).
+)*}sx
            {$return = "\n"; 1;}
        | m{((?! \*/ | /\* ).)+}sx
            {$return = ''; 1;}

GRAMMAR

...

my $decommendedText = $parse->file($input);
die "Parse failed\n" if not defined $decommendedText;
print $decommendedText;
[download]

Update: Now fixes $decommendedText being a global.