$ cat foo.pl
use strict;
use warnings;

sub new_vec { return bless { vec=>[ @_ ] }, 'flarb' };

my $a = new_vec(1, 2, 3);
my $b = new_vec(1, 4, 9);

my $t = InProduct($a, $b);
print "t=$t\n";

sub InProduct {
    my ($a, $b) = @_;

    return $a->X*$b->X + $a->Y*$b->Y + $a->Z*$b->Z;
}

package flarb;

sub X { my $self=shift; return $self->{vec}[0] };
sub Y { my $self=shift; return $self->{vec}[1] };
sub Z { my $self=shift; return $self->{vec}[2] };

$ perl foo.pl
t=36