I think it's a bug in IO::Socket::SSL. The docs for Net::SSLeay say

NOTE: Please note that SSL_alert_* function have "SSL_" part stripped from their names.

and if I call it as such then it doesn't error out for me:

$ perl -MNet::SSLeay -le 'print Net::SSLeay::SSL_alert_desc_string_lon
+g(1)'
Can't locate auto/Net/SSLeay/SSL_alert_d.al in @INC (@INC contains: /u
+sr/local/lib64/perl5 /usr/local/share/perl5 /usr/lib64/perl5/vendor_p
+erl /usr/share/perl5/vendor_perl /usr/lib64/perl5 /usr/share/perl5) a
+t -e line 1.
$ perl -MNet::SSLeay -le 'print Net::SSLeay::alert_desc_string_long(1)
+'
unknown
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