How about something like this:

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

my @known = qw( foo bar baz );

print "Talk to me, Goose.";

chomp(my $input = <STDIN>);
print "You gave me $input";

if (grep { $_ eq lc($input) } @known) {
    print ", I know about that. Have a nice day.\n";
}
else {
    print ", I don't know that, try again.\n";
}
[download]

That's possibly not super-efficient, as grep has to examine the whole list each time. If that was a concern, then you could make use of the first() function from List::Util.

Small nit: It's generally considered bad™ coding style to include multiple statements on a single line. Whitespace and newlines are quite affordable these days - use them ;)

Cheers,
Darren :)