perl -le 'my $str = "perlmonks"; print "yep" if length $str == 9'
yep
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The perlfunc manpage has a listing of categorized built-in functions which can be very helpful when one is just learning the language. The following should allow you to read it on your own system (if perl was properly installed):

    perldoc perlfunc
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Looking in the category for 'scalars and strings' we see a length function. Use perldoc again to see if that will do what desire:

    perldoc -f length
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Abigail

Your solution will miss punctuation, whitespace, and international character sets. length is the correct solution.

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

:(){ :|:&};:

Note: All code is untested, unless otherwise stated

length() was my first thought as well. Then I read the question, which specifies numbers and letters. So, missing out on punctuation, whitespace, etc, was exactly what I intended.

Abigail

This is largely a matter of interpretation. While you are probably correct that length is what the OP really wants, he did state the following:

If a string has less than eight characters (numbers/and or letters) [emphesis added]

Which would support the solution given by Abigail.