It's simply $ftp->binary according to the docs:

type (TYPE [, ARGS])
           This method will send the TYPE command to the remote
           FTP server to change the type of data transfer. The
           return value is the previous value.

ascii ([ARGS]) binary([ARGS]) ebcdic([ARGS]) byte([ARGS])
           Synonyms for "type" with the first arguments set cor-
           rectly
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