Just check @ARGV after you parse your options:

use Getopt::Long;

my $data = "file1";
my $length = 4;
my $verbose;

GetOptions (
    "length=i" => \$length, 
    "file=s"   => \$data,
    "verbose"  => \$verbose
);

if (!@ARGV) {
    print "$0: Argument required.\n";
    exit 1
}
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I also like to use POD to document my code; thus, pod2usage can be invoked to produce 'nicer' usage output:

use Pod::Usage;

...

GetOptions(
    ...
) or pod2usage(-verbose => 0);

...

# Make sure at least one argument provided
if (!@ARGV) {
    pod2usage(-verbose => 0, -message => "$0: argument required\n")
}
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