If you have a case sensitive system, that's 64 safe characters. If you can compressed (by packing numbers or otherwise) the data down to 33 bytes (floor((90/2) * (log2(64)/8))), you could use the following to convert to safe characters: 
use MIME::Base64;

sub encode {
   my ($compressed) = @_;
   my $encoded = encode_base64($compressed);
   $encoded =~ s{\+}{-}g;
   $encoded =~ s{\/}{_}g;
   return $encoded;
}

sub decode {
   my ($encoded) = @_;
   $encoded =~ s{-}{+}g;
   $encoded =~ s{_}{/}g;
   my $compressed = decode_base64($encoded);
   return $compressed;
}

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Update: On second thought, if people are gong to save these files on their own PCs, you'll need to be case-insensitive. That leaves 38 safe characters. If you wrote Base32 based on Base64 (a simple task), you'll have to compress the data down to 28 bytes (floor((90/2) * (log2(32)/8))).

Update: Fixed attrocious math.

In reply to Re^3: URL string compression? by ikegami
in thread URL string compression? by punch_card_don

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