Re^10: How to safely use $_ in a library function?

Yes, Internals::SvREADONLY(..., 1) works to correctly make a writeable variable read-only:

#! perl
use strict;
use warnings;

my $s = 'fred';
print "\$ = $s\n";
$s = 'barney';
print "\$s = $s\n";
Internals::SvREADONLY($s, 1);
$s = 'wilma';
print "\$ = $s\n";
[download]

Output:

19:04 >perl 723_SoPW.pl
$s = fred
$s = barney
Modification of a read-only value attempted at 723_SoPW.pl line 10.

19:07 >
[download]

And Internals::SvREADONLY(..., 0) then works to make the variable writeable again:

#! perl
use strict;
use warnings;

my $s = 'fred';
print "\$s = $s\n";
$s = 'barney';
print "\$ = $s\n";
Internals::SvREADONLY($s, 1);
Internals::SvREADONLY($s, 0);
$s = 'wilma';
print "\$ = $s\n";
[download]

Output:

19:10 >perl 723_SoPW.pl
$s = fred
$s = barney
$s = wilma

19:12 >
[download]

But it doesn’t work on a variable which is aliased to a literal:

#! perl
use strict;
use warnings;

for my $s ('fred')
{
    print "\$s = $s\n";
    Internals::SvREADONLY($s, 0);
    $s = 'wilma';
    print "\$ = $s\n";
}
[download]

Output:

19:14 >perl 723_SoPW.pl
$s = fred
Modification of a read-only value attempted at 723_SoPW.pl line 9.

19:16 >
[download]

So Internals::SvREADONLY() “works” under Windows; but (it would appear) its effect on an aliased variable is not portable?

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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