$rule->name(length >= 50);

That should be $rule->size(length >= 50) or better yet:

$rule->exec(sub {length >= 50});
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Here's an example of what I think that you are trying to do:

#!/usr/bin/perl

use strict;
use warnings;
use File::Find::Rule;

open(STDOUT, '>', 'size.txt');

my $rule =  File::Find::Rule->new;
$rule->file;
$rule->name( '*.pm' );
$rule->exec(sub {length >= 25});

my @files = $rule->in( @INC );
foreach my $file(@files){
    print $file, "\n";
}

exit;
[download]