When you do a command like this:

$ command
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Then the system looks at the diretories in you path and executes the first matching one. To see your path do:

$ echo $PATH
/bin:/sbin:/usr/bin:/usr/sbin:
$
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In this case we have 4 dirs in the PATH. The OS will look for 'command' in /bin, then /sbin....etc until it finds an exe called 'command' If there is one in /bin it will never look any further.

You are seeing the result of this behaviour. You will have (prabably a symlink) to 'perl' in /usr/bin so that is the first one found, and the one that gets executed when you ask for perl. If you do '/usr/local/bin/perl' (probable install location) you will get 5.8. Either call 5.8 on the shebang line as shown or replace the symlink in /usr/bin.....

To find all the perls on your system do:

$ locate -u
$ locate bin/perl
/usr/bin/perl
/usr/local/bin/perl
$
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The -u flag to locate updates the DB (may take a minute, background with &, add a daily cron job...) and then we run it to find all the things on the system that match the search string.