It's indeed possible to bend the parser in a way that it thinks BEGIN and family are subs

use strict;
use warnings;
use Keyword::Simple;

sub no_begin ($&){
    warn "no_begin(@_) called";
}

my @code;

BEGIN{
    my @compile_blocks = qw(BEGIN UNITCHECK CHECK INIT END);

    for my $block (@compile_blocks) {

        # bend parser
        Keyword::Simple::define $block, sub {
            my ($ref) = @_;
            substr($$ref, 0, 0) = "no_begin '$block', sub";
        };

        # test code
        push @code , <<__CODE__;
        $block { die "owened by $block" }
__CODE__

    }
}

BEGIN { die "owened by BEGIN" };
UNITCHECK { die "owened by UNITCHECK" };
CHECK { die "owened by CHECK" };
INIT { die "owened by INIT" };
END { die "owened by END" };

eval join "\n", @code;
[download]

-*- mode: compilation; default-directory: "d:/tmp/pm/" -*-
Compilation started at Wed Oct  6 19:04:01

C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/KW_simple_regex_BEGIN.pl 
no_begin(BEGIN CODE(0x694268)) called at d:/tmp/pm/KW_simple_regex_BEG
+IN.pl line 6.
no_begin(UNITCHECK CODE(0x6556d0)) called at d:/tmp/pm/KW_simple_regex
+_BEGIN.pl line 6.
no_begin(CHECK CODE(0x6942b0)) called at d:/tmp/pm/KW_simple_regex_BEG
+IN.pl line 6.
no_begin(INIT CODE(0x6c8aa0)) called at d:/tmp/pm/KW_simple_regex_BEGI
+N.pl line 6.
no_begin(END CODE(0x6c8c38)) called at d:/tmp/pm/KW_simple_regex_BEGIN
+.pl line 6.
Bareword found where operator expected at (eval 5) line 5, near "}

        CHECK"
    (Missing operator before CHECK?)
[download]

But unfortunately does evaling the code not catch parsing errors anymore... (reason here BEGIN{} blocks don't need a trailing semicolon)

so the answer is:

• Yes BEGIN* blocks can be disabled.
• But this is best done in an extra process