There are several theories about how to tell when a builtin is overridable, for example if prototype 'CORE::foo' is defined foo is overridable. That's mostly true, but there are exceptions.

Here's a quick way to tell:

$ perl -e'BEGIN{ *CORE::GLOBAL::use = sub{1};} use CGI qw/:standard/; 
+print header'
Content-Type: text/html; charset=ISO-8859-1

$
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A simpler version:

$ perl -e'use Fatal qw/use/;'
Cannot make a non-overridable builtin fatal at /usr/lib/perl5/5.8.4/Fa
+tal.pm line 108.
BEGIN failed--compilation aborted at -e line 1.
$
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