...
use CGI qw/:standard/;
my $q = CGI->new();
my $level = $q->param('level');

my %run_level = ('foo'      =>  \&foo,
                 'bar'      =>  \&bar,
                 'default'  =>  \&default
                );

if (exists $run_level{$level}) {
  &{ $run_level{$level} };
} else {  
  default();
}

sub foo {
  return "Sub foo called";
}
sub bar {
  return "Sub bar called";
}
sub default {
  return "Sub default called";
}

##</code><code>##

default() unless exists $run_level{$level}; 
exit unless exists $run_level{$level}; # or add exit into default()
&{ $run_level{$level} };