Tcl Requires Chilkat v11.7.0+
Box: Download Binary Content into BinData

Run the reusable setup once per HttpCurl object, then select names and execute the operation. The setup is repeated here only to make this published example independently runnable. Download a small file into memory. For large files use the direct-to-disk example.
See the Box API guide for setup and the complete workflow.
Chilkat Tcl Downloads

Download Chilkat for Tcl
Tcl

load ./chilkat.dll

set success 0

#  Requires Chilkat 11.7.0 or later and a previously unlocked Chilkat bundle.
#  See Global Unlock Sample.
#  Guide: Box API Guide.

set success 0

#  BEGIN ONE-TIME SETUP
#  This block is repeated so each published example can run on its own.
#  In an application, run the setup example once and pass the SAME curl object
#  to operation code below. Do not recreate it for every operation.
#  See Reusable Box HttpCurl Setup.
set curl [new_CkHttpCurl]

CkHttpCurl_SetVar $curl "access_token" "BOX_ACCESS_TOKEN"
CkHttpCurl_SetVar $curl "parent_name" "/"
#  Box's fixed root ID is an implementation detail; callers supply names.
CkHttpCurl_SetVar $curl "parent_id" "0"
set setupCommand [new_CkStringBuilder]

#  Register findParent; registration sends no request.
CkStringBuilder_SetString $setupCommand "curl -X GET \"https://api.box.com/2.0/folders/0/items?limit=1000&fields=id,type,name\" -H \"Authorization: Bearer ACCESS_TOKEN\""
set setupReplacements [CkStringBuilder_Replace $setupCommand "ACCESS_TOKEN" [CkHttpCurl_getVar $curl "access_token"]]
set success [CkHttpCurl_AddFunction $curl "findParent" [CkStringBuilder_getAsString $setupCommand]]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

#  Register findFolder; registration sends no request.
CkStringBuilder_SetString $setupCommand "curl -X GET \"https://api.box.com/2.0/folders/{{parent_id}}/items?limit=1000&fields=id,type,name\" -H \"Authorization: Bearer ACCESS_TOKEN\""
set setupReplacements [CkStringBuilder_Replace $setupCommand "ACCESS_TOKEN" [CkHttpCurl_getVar $curl "access_token"]]
set success [CkHttpCurl_AddFunction $curl "findFolder" [CkStringBuilder_getAsString $setupCommand]]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

#  Register findFile; registration sends no request.
CkStringBuilder_SetString $setupCommand "curl -X GET \"https://api.box.com/2.0/folders/{{folder_id}}/items?limit=1000&fields=id,type,name\" -H \"Authorization: Bearer ACCESS_TOKEN\""
set setupReplacements [CkStringBuilder_Replace $setupCommand "ACCESS_TOKEN" [CkHttpCurl_getVar $curl "access_token"]]
set success [CkHttpCurl_AddFunction $curl "findFile" [CkStringBuilder_getAsString $setupCommand]]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

#  Register findDestination; registration sends no request.
CkStringBuilder_SetString $setupCommand "curl -X GET \"https://api.box.com/2.0/folders/0/items?limit=1000&fields=id,type,name\" -H \"Authorization: Bearer ACCESS_TOKEN\""
set setupReplacements [CkStringBuilder_Replace $setupCommand "ACCESS_TOKEN" [CkHttpCurl_getVar $curl "access_token"]]
set success [CkHttpCurl_AddFunction $curl "findDestination" [CkStringBuilder_getAsString $setupCommand]]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

#  Register getFileDetails; registration sends no request.
CkStringBuilder_SetString $setupCommand "curl -X GET \"https://api.box.com/2.0/files/{{file_id}}?fields=id,sha1,file_version\" -H \"Authorization: Bearer ACCESS_TOKEN\""
set setupReplacements [CkStringBuilder_Replace $setupCommand "ACCESS_TOKEN" [CkHttpCurl_getVar $curl "access_token"]]
set success [CkHttpCurl_AddFunction $curl "getFileDetails" [CkStringBuilder_getAsString $setupCommand]]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

#  Register getEnterprise; registration sends no request.
CkStringBuilder_SetString $setupCommand "curl -X GET \"https://api.box.com/2.0/users/me?fields=enterprise\" -H \"Authorization: Bearer ACCESS_TOKEN\""
set setupReplacements [CkStringBuilder_Replace $setupCommand "ACCESS_TOKEN" [CkHttpCurl_getVar $curl "access_token"]]
set success [CkHttpCurl_AddFunction $curl "getEnterprise" [CkStringBuilder_getAsString $setupCommand]]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

#  Register output rules once. Chilkat 11.7.0 evaluates {{name}} variables
#  when extracting each output, using the current value at that time.
set success [CkHttpCurl_AddOutput2 $curl "findParent" "entries" "name" "{{parent_name}}" 1 "id" "parent_id"]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

set success [CkHttpCurl_AddOutput2 $curl "findFolder" "entries" "name" "{{folder_name}}" 1 "id" "folder_id"]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

set success [CkHttpCurl_AddOutput2 $curl "findFile" "entries" "name" "{{file_name}}" 1 "id" "file_id"]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

set success [CkHttpCurl_AddOutput2 $curl "findDestination" "entries" "name" "{{destination_name}}" 1 "id" "destination_id"]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

set success [CkHttpCurl_AddOutput $curl "getFileDetails" "file_version.id" "file_version_id"]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

set success [CkHttpCurl_AddOutput $curl "getFileDetails" "sha1" "file_sha1"]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

set success [CkHttpCurl_AddOutput $curl "getEnterprise" "enterprise.id" "enterprise_id"]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    exit
}

#  END ONE-TIME SETUP
#  The name lookups below assume each listing fits in one page (1000 items).
#  Parent and destination names select root children; folder_name selects a child
#  of parent_name. "/" selects the root parent. These are not arbitrary paths.

#  BEGIN OPERATION -- keep the configured object when reusing it.
#  Inputs are names. Retain cached IDs when the selected names have not changed.
set parentName "/"
set folderName "images"
set fileName "penguins.jpg"
if {expr [[CkHttpCurl_getVar $curl "parent_name"] != $parentName]  ||  [expr [$parentName == "/"]  &&  [[CkHttpCurl_VarDefined $curl "parent_id"] == 0]]} then {
    CkHttpCurl_SetVar $curl "parent_name" $parentName
    CkHttpCurl_ClearVar $curl "parent_id"
    CkHttpCurl_ClearVar $curl "folder_id"
    CkHttpCurl_ClearVar $curl "file_id"
    CkHttpCurl_ClearVar $curl "file_version_id"
    CkHttpCurl_ClearVar $curl "file_sha1"
    if {$parentName == "/"} then {
        CkHttpCurl_SetVar $curl "parent_id" "0"
    }

}

if {[CkHttpCurl_getVar $curl "folder_name"] != $folderName} then {
    CkHttpCurl_SetVar $curl "folder_name" $folderName
    CkHttpCurl_ClearVar $curl "folder_id"
    CkHttpCurl_ClearVar $curl "file_id"
    CkHttpCurl_ClearVar $curl "file_version_id"
    CkHttpCurl_ClearVar $curl "file_sha1"
}

if {[CkHttpCurl_getVar $curl "file_name"] != $fileName} then {
    CkHttpCurl_SetVar $curl "file_name" $fileName
    CkHttpCurl_ClearVar $curl "file_id"
    CkHttpCurl_ClearVar $curl "file_version_id"
    CkHttpCurl_ClearVar $curl "file_sha1"
}

CkHttpCurl_ClearTargetOutput $curl "*"
set authorizedCommand1 [new_CkStringBuilder]

CkStringBuilder_SetString $authorizedCommand1 "curl -X GET \"https://api.box.com/2.0/files/{{file_id}}/content\" -H \"Authorization: Bearer {{access_token}}\" -L"
set headerReplacement1 [CkStringBuilder_Replace $authorizedCommand1 "{{access_token}}" [CkHttpCurl_getVar $curl "access_token"]]
set success [CkHttpCurl_DoYourThing $curl [CkStringBuilder_getAsString $authorizedCommand1]]
if {expr [$success != 1]  ||  [[CkHttpCurl_get_StatusCode $curl] != 200]} then {
    puts "Box request failed. HTTP status: [CkHttpCurl_get_StatusCode $curl]"
    puts "Failure reason: [CkHttpCurl_get_FailReason $curl]"
    puts [CkHttpCurl_responseBodyStr $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    delete_CkStringBuilder $authorizedCommand1
    exit
}

set data [new_CkBinData]

set success [CkHttpCurl_GetResponseBd $curl $data]
if {$success != 1} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $setupCommand
    delete_CkStringBuilder $authorizedCommand1
    delete_CkBinData $data
    exit
}

puts "Downloaded bytes: [CkBinData_get_NumBytes $data]"

delete_CkHttpCurl $curl
delete_CkStringBuilder $setupCommand
delete_CkStringBuilder $authorizedCommand1
delete_CkBinData $data