I'm not a devel expert, but here is some code posted awhile back.

#!/usr/bin/perl
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" }
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