Since you don't seem to care about what the casing was that the user entered, why not just upper (or lower) case it?

my @known = qw/FOO BAR BAZ/;
print "Talk to me, Goose. ";

my $input = <STDIN>; chomp($input);

$input = uc $input ;

if( grep { $input eq $_ } @known ){
    print "You gave us $input, which is known. Have a nice day.\n";
}else{
    print "bad input: $input\n";
}
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