use strict;
use warnings;

sub remember {
    my $memory if 0;
    print "Previous value of \$memory: $memory\n";
    $memory = shift;
}

remember "foo";
remember 42;
remember "bar";
remember;

__END__
Use of uninitialized value in concatenation (.) or string at - line 6.
Previous value of $memory: 
Previous value of $memory: foo
Previous value of $memory: 42
Previous value of $memory: bar