Are you sure you didn't code it as:

Encryption => 'EXP_CRYPT',
or
Encryption => "EXP_CRYPT",
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Looking at the module's source, I think it needs to be a bareword Encryption => EXP_CRYPT (ie no quotes).
And the error message you've supplied is consistent with EXP_CRYPT having been quoted, and inconsistent with EXP_CRYPT having been unquoted.

The error arises from this check in FTPSSL.pm:

  return _croak_or_return (undef, $die, $dbg_flg,
                           "Encryption mode unknown!  ($encrypt_mode)"
+)
      if ( $encrypt_mode ne IMP_CRYPT && $encrypt_mode ne EXP_CRYPT &&
           $encrypt_mode ne CLR_CRYPT );
[download]

Near the beginning of FTPSSL.pm we have:

use constant EXP_CRYPT => "E";
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Therefore, if FTPSSL.pm's $encrypt_mode has been assigned as you intend, it should have been set to E, not EXP_CRYPT as the error reports.

At least that's the way it looks to me.

Cheers,
Rob

UPDATE: Alternatively, it might be that you haven't imported EXP_CRYPT - in which case that could be fixed by specifying:

Encryption => Net::FTPSSL::EXP_CRYPT,
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Without seeing more of your code, it's hard to know for sure.
(Also fixed typos in my initial post.)