The cheating way of doing this is to observe the symbols table of package main before and after importing the symbols for ':form' :

use CGI;
                                                                      
+          
my %symbols;
                                                                      
+          
for my $foo ( keys %main:: )
{
   $symbols{$foo} = 1;
}
                                                                      
+          
CGI::import 'CGI',':form';
                                                                      
+          
for my $foo ( grep !exists $symbols{$_} , keys %main:: )
{
   print $foo,"\n";
}
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/J\