for my $key (keys %hash){
  my $outputfile = "output_$key";
  $outputfile =~ s{/}{-}g;
  $outputfile = join '/', $str,$outputfile; # <---- this is missing
  print "Writing to $outputfile\n";
  open OUT,'>',$outputfile 
       or die "Could not open $outputfile : $!";
  print OUT $hash{$key};
  close OUT;
}