You're making $key too long by a factor of 39. You want something more like this (where @encoded is the given byte array):

my $encodedString = pack('c*', @encoded);
my $key = 'Copyright © 2000 ActiveState Tool Corp.';
$key = $key x ( length($encodedString) / length($key) + 1);
$key = substr($key, 0, length($encodedString));
print $key ^ $encodedString;
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