package Tie::Scalar::Yell;

sub TIESCALAR { bless \my $x, shift }
sub STORE { ${ $_[0] } = $_[1] }

sub FETCH {
    warn "accessed\n";
    return ${ $_[0] };
}

package main;

tie my $x, 'Tie::Scalar::Yell';

$x = 'yeller';
print "yeller: $x\n";  # accessed here
copier( $x );

sub copier {
    my $arg = shift;   # accessed here
    print "my arg once: $arg\n";  # ...or here
    print "my arg twice: $arg\n"; # here?
}
__END__
accessed
yeller: yeller
accessed
my arg once: yeller
my arg twice: yeller