# First declare your file handle
my $OUT;
...
foreach ... {
   while ... {
      if ... {
         # Error found, so...

         # open the file if not open yet
         if (!defined $OUT) {
            open $OUT, '>', "$path/$file" or die "Uggabug! $!";
         }

         # and log the error
         print $OUT "Yer error message\n";
      }
   }
}
# finally, close it if you've opened it
close($OUT) if defined $OUT;