# heres a functional way
my @no_undefs=map{$_ || 0} @has_undefs;

# or with a modifier
!defined($array[$_]) and $array[$_]=0
       foreach 0..$#array;

# or more elegantly
$array[$_]||=0 foreach 0..$#array;

# or with a funky sub
# might be slow or big lists though
sub undefs2zero {$_[$_]||=0 foreach 0..$#_} 
undefs2zero(@array);