use warnings;
use strict;
use Getopt::Long;

my @in;
GetOptions('input=s', \@in) or die;
die "Error: cant have multiple inputs" if @in > 1;
print "input = @in\n";
[download]

UPDATE: There is another solution at: http://stackoverflow.com/questions/10045067/can-getoptlong-getoptions-generate-an-error-if-the-same-option-occurs-multiple

That's exactly what I'm trying to avoid. I need to end the script rather than overwriting by the last appearance of the param.

That goes against the general philosophy of running stuff from a shell command line and allowing users to customize their defaults. Users are free to set up aliases with their most commonly used flags and allowing duplicate flags / taking that last flag allows them to still access all the command line options. Flags for default options exist for this reason.

See also Getopt::Flex, Getopt::Valid, FormValidator::Simple, CGI, App::Cmd, App::CLI

Kind of silly , but you could write

#!/usr/bin/perl --
use strict; use warnings;
use Data::Dump;
use CGI;
use Getopt::Long;
use FormValidator::Simple;
Fudge(qw[ -i 3 --input=6 --input=9 ]);
Fudge('-i', '__FILE__' );

sub Fudge {
    local @ARGV = @_;
    my $query = CGI->new({});
    my $queryer = sub {
       my( $name, $value ) = @_;
       my @vals = $query->param( $name );
       $query->param( $name, $value, @vals );
       return;
    };
    GetOptions(
    'input|i=s' => $queryer,
    );

    my $rules = [
        input => [ qw[ NOT_BLANK ASCII SINGLE ], ['REGEX' => qr/^\w+$/
+], ],
    ];

    my $results = FormValidator::Simple->check( $query => $rules );

    if ( $results->has_error ) {
        foreach my $key ( @{ $results->error() } ) {
            dd $results->record( $key )->data;
            foreach my $type ( @{ $results->error($key) } ) {
                print "invalid: $key - $type \n";
            }
            print "\n";
        }
    } else {
        print "valid input: ", $results->record('input')->data, "\n";
    }
}

sub FormValidator::Simple::Validator::SINGLE {
    package FormValidator::Simple::Validator;
    my ($self, $params, $validatorArgs) = @_;
    my $data     = $params->[0];
    my $selected = ref $data ? $data : [$data];
    return scalar(@$selected) == 1 ? TRUE : FALSE;
}



__END__
[9, 6, 3]
invalid: input - SINGLE
invalid: input - REGEX

valid input: __FILE__
[download]

SINGLE is "monkeypatched" instead of a plugin