I have copy the mail.pl from RHEL3 server to another RHEL4 server , but when I run mail.pl , it pop that below error , can advise how can I fix it ? thanks.

Can't locate Mail/Sender.pm in @INC (@INC contains: /usr/lib/perl5/5.8.5/i386-li
nux-thread-multi /usr/lib/perl5/5.8.5 /usr/lib/perl5/site_perl/5.8.5/i386-linux-
thread-multi /usr/lib/perl5/site_perl/5.8.5 /usr/lib/perl5/site_perl /usr/lib/pe
rl5/vendor_perl/5.8.5/i386-linux-thread-multi /usr/lib/perl5/vendor_perl/5.8.5 /
usr/lib/perl5/vendor_perl/5.8.3/i386-linux-thread-multi /usr/lib/perl5/vendor_pe
rl/5.8.3 /usr/lib/perl5/vendor_perl .) at /usr/local/bin/mail.pl line 5.

Its telling you it can't locate the MAIL::Sender Perl module in any of the usual places.
Given that both versions 3 & 4 of RHEL are out of support, I think you're going to struggle.
If you are prepared to pay for Extended Update Support, RH will help with the RHEL4
http://www.redhat.com/security/updates/errata/

I would try to get that module, type as root:
cpan Mail::Sender