# our $Zeropack = "\0" x 4;
# our $MaxNum32bit = 2 ** 32;
# Use:                                     ## n - number of 64bit pack
+ed numbers in string
# my @Numbers = UnPack( n, $stringpack );  ## string input, array outp
+ut
sub UnPack
{  my $todo = shift; my $input = shift;
   my @output = (); my $arno = 0;
   while( $arno < $todo )
   {   my $pack = substr( $input, $arno * 8, 8 );
       if ( substr($pack,0,4) eq $Zeropack )
       {   $output[$arno] = unpack("N", substr( $pack, 4, 4 ) );
       }
       else
       {   my ( $upper, $lower ) = unpack( "N N", $pack );
           if ( ! defined $upper ) { $upper = 0; }
           if ( ! defined $lower ) { $lower = 0; }
           $output[$arno] = ( $upper * $MaxNum32bit ) + $lower ;
       }
       $arno++
   }
   return ( @output );
}
[download]

You don't verify that the input is correct and you are doing too much work for such a simple task.    I would do something like this:

# use constant Max_Num_32_bit => 2 ** 32;
# Use:                                     ## n - number of 64bit pack
+ed numbers in string
# my @Numbers = UnPack( n, $stringpack );  ## string input, array outp
+ut

sub UnPack {
    my ( $todo, $input ) = @_;
    if ( @_ != 2 || length $input < $todo * 8 ) {
        warn "Invalid input for UnPack()\n";
        return;
        }
    my @output;
    for my $arno ( 0 .. $todo - 1 ) {
        my $pack = substr $input, $arno * 8, 8;
        my ( $upper, $lower ) = unpack 'N N', $pack;
        push @output, $upper * Max_Num_32_bit + $lower ;
        }
    return @output;
    }
[download]