Another way:

c:\@Work\Perl\monks>perl -wMstrict -le
"use List::Util qw(sum);
 use Regexp::Common;
 ;;
 my $user_total = total(<STDIN>);
 ;;
 print qq{The total of user-input integers is $user_total};
 ;;
 sub total {
   ;;
   my $n = $RE{num}{real};
   ;;
   die qq{user input <<@_>> unuseable}
     unless @_ == map m{ \A \s* $n (?: \s+ $n)* \s* \z }xms, @_;
   ;;
   return sum map m{ $n }xmsg, @_;
   }
"
123
4. .5 0.0 .0 0. -.7
-11
^Z
The total of user-input integers is 115.8
[download]

(On Windoze: Must use ctrl-Z to terminate input.) (Good luck with your homework.)

Update: Here's yet another and, I think, better way. A single join is almost certainly going to be more efficient than two map-s; I don't know what I was thinking! (Minimally tested.)

sub total {
  my $u_i = join '', @_;
  my $n = $RE{num}{real};
  ;;
  die 'user input >>', @_, '<< unuseable'
    unless $u_i =~ m{ \A \s* $n (?: \s+ $n)* \s* \z }xms;
  ;;
  return sum $u_i =~ m{ $n }xmsg;
  }
[download]

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