use Devel::Peek;

my $str = 'the key is:';
my $key = 'hello';
Dump($key);
decrypt( $str, $key );
$key = 'gone!';
# $key = 'x'x100000;   # a string that won't fit will change PV
Dump($key);
decrypt( $str, $key );

sub decrypt { warn "\nGot @_\n\n" }

__DATA__
SV = PV(0x15d529c) at 0x1a8460c
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x1a4a8ac "hello"\0
  CUR = 5
  LEN = 6

Got the key is: hello

SV = PV(0x15d529c) at 0x1a8460c
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x1a4a8ac "gone!"\0
  CUR = 5
  LEN = 6

Got the key is: gone!