For true hashes, yes. Magical hashes can extend this.

Could I save memory by storing the MIME::Base64 version of the integer key? (Or perhaps using pack to create a 4-byte or 8-byte string.)

$ perl -MMIME::Base64 -E'say length for
   $ARGV[0],
   encode_base64($ARGV[0]),
   pack("N", $ARGV[0]),
   pack("Q", $ARGV[0])
' 1234
4
9
4
8
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