If you want to add it permanently then, assuming you use sh (or a descendant thereof but not csh or any of its variants), add the following into your profile i.e. $HOME/.profile

PERL5LIB=$PERL5LIB export PERL5LIB<new path>
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or possibly

export PERL5LIB=$PERL5LIB
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If you want all users to inherit this setting, then add the above into /etc/profile.

Note that the above will vary if you are unfortunate/misguided enough to use the C shell - if nothing else, the file to change will be $HOME/.login.

Note also that you should avoid using shell specific scripts e.g. .bashrc, .kshrc or even .cshrc to set login session environment variables - these should be used merely to modify the login shell environment on a per shell basis.