Really? I get this...

#!/usr/bin/env perl
use strict;
use warnings;

for (qw(literal1 literal2 literal3))
{
    print "$_\n";
    foo('wilma');
}

sub foo
{
    Internals::SvREADONLY($_, 0);
    $_ = 'fred';
    goto &bar;
}

sub bar
{
    my ($name) = @_;
    print "foo --> name = $name, \$_ = $_\n";
}

__END__
literal1
foo --> name = wilma, $_ = fred
literal2
foo --> name = wilma, $_ = fred
literal3
foo --> name = wilma, $_ = fred
[download]

You can see what Internals functions you have like this:

perl -E'say for sort keys %Internals::'
[download]

Many of these built-in functions, including stuff in the Internals, utf8, re and UNIVERSAL packages amongst others, are defined in the file universal.c in the Perl source code. Search in the file for the string "xsub_details" and you will find a list. That's not a comprehensive list though; Internals::V and mro::get_linear_isa are certainly missing.