1:35 >perl -E "say for sort keys %Internals::"
SvREADONLY
SvREFCNT
V
hv_clear_placeholders

 1:35 >

##</code><code>##

#! perl
use strict;
use warnings;
use Internals qw(SetReadWrite);

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

sub foo
{
    SetReadWrite(\$_);
    $_ = 'fred';
    goto &bar;
}

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

##</code><code>##

12:26 >perl 723_SoPW.pl
literal1
Modification of a read-only value attempted at 723_SoPW.pl line 15.

12:28 >

##</code><code>##

#! perl
use strict;
use warnings;
use Internals qw(IsWriteProtected);

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

sub foo
{
    if (IsWriteProtected(\$_))
    {
        warn "Can't modify \$_";
    }
    else
    {
        $_ = 'fred';
    }

    goto &bar;
}

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

##</code><code>##

12:28 >perl 723_SoPW.pl
literal1
Modification of a read-only value attempted at 723_SoPW.pl line 20.

12:36 >