or you can have your FindNewerThan sub return a sub:

sub findNewerThan {
  my $days_ago = days(shift);
  return sub {
    my $filename = $File::Find::name;
    return unless -f $filename;

    #...do something with $days_ago

    push(@files, $filename);
  }
}

find( findNewerThan(5), $file_dir_root );
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