gistfile1.pl

package Foo::Handle;
use strict;
use warnings;

sub TIEHANDLE {
    my ($class, $handle) = @_;
    bless \$handle, $class;
}

sub PRINT    {
    my ($self, $buff) = @_;
    $buff .= $\; # if omit this line then restore $\.
}

package main;
use strict;
use warnings;
use feature 'say';
use Symbol qw(gensym);

open my $fh, '>', 'FOO';
printf "[%s]\n", $\;
say $fh 'foo';
printf "[%s]\n", $\; # $\ is restored

my $gh = gensym;
tie *$gh, 'Foo::Handle';
printf "[%s]\n", $\;
say $gh 'foo';
printf "[%s]\n", $\; # $\ is not restored (it's bug?)

__END__
$ ./foo.pl
Use of uninitialized value $\ in printf at foo.pl line 23.
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Use of uninitialized value $\ in printf at foo.pl line 25.
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Use of uninitialized value $\ in printf at foo.pl line 29.
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