use CGI qw(header start_html Vars pre);
use Data::Dumper;

print
    header(),
    start_html();

my $ref = Vars();
print pre( Dumper( $ref ) );

# or just fully qualify it                                                             
use CGI ":standard";

# ...                                                                                  
my %params = CGI::Vars();

print pre( Dumper( \%params ) );

# or what the POD suggests
use CGI qw(:standard :cgi-lib);

print pre( Dumper( [ Vars() ] ) );