I need help with problem...

i need to create a program that have two subroutines 
add() and multiply() that will add or 
multiply its arguments and print the 
result in screen. 

this is my current code:

if ($ARGV[0] =~ /add/) { 
$rtn = add($ARGV[1]); 
print "The sum is: $rtn";
} elsif ($ARGV[0] =~ /multiply/) { 
$rtn = multiply($ARGV[1]); 
print "The product is: $rtn";
} 
sub add { 
 
}
sub multiply { 
 
} 


can anybody fill me in this code..im kinda confused..
thanks for your help guys..