This will get you started:

#!/usr/bin/perl -w
use strict;

my $string = 'just another perl hacker';

# generate a lookup hash containing the words in our language
# as the keys, we set the values to 1 with the ++ syntax
# so as to define the keys which are all we use
my %lang;
do{ chomp; $lang{$_}++ }for <DATA>; 

# split the test string on whitespace to give us an
# array that will contain all the 'words' where a 
# word is a character sequence
my @bits = split /\s/, $string;

# iterate over our word array seeing if they are 
# defined in our langugue specification
for (@bits) {
    die "Word '$_' not in language!\n" unless defined $lang{$_};
}

# if we have not died then all the words are OK
print "Success, \$string only contains words in language!\n";

__DATA__
just
another
finite
automaton
perl
hack
[download]

doc

print(s<>ecode?scalar reverse :p)

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