sub invalid_line {
    my ($line) = @_;
    ...
    return 1 if $invalid;
}

my @files;
# todo, get list of files

for my $file (@files) {
    open FILE '<', $file;
    if (scalar grep { invalid_line($_) }
               grep { $_ =~ /^$keyword/ }
               <FILE>
    ){
        # file is invalid
    } else {
        # file is valid
    }
    close FILE;
}