sub total {
 my $number;

 foreach (@_) {
  # you are allowing input from STDIN which is going to
  # add a \n on the end. So you're going to have to add
  # a chomp
  chomp;  # UPDATE: ysth is correct, but I'll leave it in
          # for the next line I added 
  next if (!/^[+-]?[[:digit:].]+$/); # UPDATE: This is slightly better

  # $number += @_[$_];
  # $_ is assigned the value of the element in @_ not the index
  # So you want
  $number += $_;
  
 }

 return $number;
}

my @fred = qw{ 1 3 5 7 9 };
my $fred_total = &total(@fred);  # & not required
print "The total of \@fred is $fred_total.\n";
print "Enter some numbers on separate lines: \n";
my $user_total = &total(<STDIN>);
print "The total of those numbers is $user_total.\n\n";