The program "email-reminder-2.0.pl" appears to run correctly when called independently but not when called by cron.

It therefore seems that a different @INC is being used when cron runs the script.
The error message plainly states that @INC is:

/usr/local/lib64/perl5/5.30
/usr/local/share/perl5/5.30
/usr/lib64/perl5/vendor_perl
/usr/share/perl5/vendor_perl
/usr/lib64/perl5
/usr/share/perl5
[download]

If /usr/local/share/perl5/Mail/Mailer.pm exists, then @INC needs to include /usr/local/share/perl5.
But /usr/local/share/perl5 is not one of the locations specified in @INC when cron runs the script.

Cheers,
Rob