(each separate char)     # The string split into a list of chars

    map{ ord }               # The ordinal value of the char in $_

    map{                     # uses $b to buffer the 4 bits from the previous 
                             # character
                            
      $\ .=                  # Append to $\...
        chr(                 # the character with ordinal value of
          96+(               # the 96 ( 01100000 ) that was removed plus
            $b               # the buffered bits from the previous char
                             # stored in $b's 4 high bits (11110000)
              |              # Fill the four low bits with
                $_ >> 4 & 63 # the four high bits from the current character
            || 0xc0          # If zero (ie space), use 192 (11000000) instead
          )
          &255               # for space, 192 + 96 = 288 (100100000), so remove 
                             # the 9th bit which leaves 32 (00100000)
      );
      $b =                   # buffer the 4 low bits of $_ as the 4 high bits 
                             # $b by:
           $_<<8>>4          # equiv of $_ << 4 (ie shift $_ left by 4)
            & 0xf0           # and zero all but bits 5-8
    }