use strict;
use CGI; # or CGI::Simple

my %dispatch = (
  runmode1 => 'My::Package1',
  runmode2 => 'My::Package2',
  runmode3 => 'My::Package3',
);

my $rm = $q->param('rm');
eval "require $dispatch{$rm}" or die "runmode handler not found";
$dispatch{$rm}->handler($q); 

##</code><code>##

sub handler
{
    shift; # ignore 1st arg (package name)
    my $q = shift;
    ...
    # do what ever runmode requires
    # return html output
}