If you want to exclude a directory tree from a File::Find search, the best way is to prune the search space. This way you don't have to needlessly grovel through the directories and say "Should I do something here? Nope." The first time you encounter the directory you want to tell File::Find to short-circuit the search and go onto the next object in the filesystem. The following snippet should give you an idea about how to go about it:

#! /usr/bin/perl -w

use strict;
use File::Find;

my $ignore = 'winnt';
find(
  sub {
    if( -d $_ and m{$ignore$}o ) {
      $File::Find::prune = 1;
      return;
    }
    print "$File::Find::name\n" if -f _;
  },
  shift || '.'
);
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Tailor to suit your circumstances. You may have to worry about case-sensitivity of filenames (or not). You may also want to perform an exact match with eq instead of performing a regex match. You may want to exclude more than one directory.