You got a reply which you already had answered. Why? Because you didn't use the <code> tags.

$ which perl
/usr/local/bin/perl
$ /usr/local/bin/perl -v
This is perl, v5.8.6 ...
$ /usr/bin/perl -v
This is perl, v5.8.5 ...
$ perl -v
This is perl, v5.8.5 ...
[download]

Another cause might be aliases in your .profile / .bashrc / .cshrc file, which would mask the word 'perl' in one command but not in others.