Can CGI have different environment variables than the default environment variables provided by the server?

Yes and no. Depends on what you mean by different. As a whole, the lists of environment variables are different. Some variables are common to both, while others are unique.

#!/usr/bin/perl -w

use strict;
use warnings;

my $i = 1;

for (sort keys %ENV){
    print "$i $_\n";
    $i++;
}
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On my current system these environment varibles were specific to the command line:

APPDATA
HOMEDRIVE
HOMEPATH
LOGONSERVER
PROMPT
SESSIONNAME
SWIFT_TRACE
SYSTEMDRIVE
SYSTEMROOT
TEMP
TMP
USERDNSDOMAIN
USERDOMAIN
USERNAME
USERPROFILE
VS90COMNTOOLS
WF_RESOURCES
WINDIR
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And these were specific to the webserver:

CONTENT_LENGTH
GATEWAY_INTERFACE
HTTPS
HTTP_ACCEPT
HTTP_ACCEPT_ENCODING
HTTP_ACCEPT_LANGUAGE
HTTP_CONNECTION
HTTP_HOST
HTTP_USER_AGENT
INSTANCE_ID
LOCAL_ADDR
PATH_INFO
PATH_TRANSLATED
REMOTE_ADDR
REMOTE_HOST
REQUEST_METHOD
SCRIPT_NAME
SERVER_NAME
SERVER_PORT
SERVER_PORT_SECURE
SERVER_PROTOCOL
SERVER_SOFTWARE
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And these are common to both:

ALLIANCE
ALLUSERSPROFILE
ARCH
COMMONPROGRAMFILES
COMPUTERNAME
COMSPEC
CSYS_DCE_CLIENT_PROT
CVSROOT
DCE_CLIENT_PROT
DCE_SERVER_PROT
ERRLOG_CATALOG_DIR
FP_NO_HOST_CHECK
NUMBER_OF_PROCESSORS
OAPI_TRACE
OAPI_TRACE_INSTANT_FLUSH
OS
PATH
PATHEXT
PROCESSOR_ARCHITECTURE
PROCESSOR_IDENTIFIER
PROCESSOR_LEVEL
PROCESSOR_REVISION
PROGRAMFILES
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