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);