I guess something like this? I would probably return just a 0 or 1 for a true/false value. Here "True" and "False" just happened so the printout looked nice.

#!/usr/bin/perl
use strict;
use warnings;


my $x = isABN("51 824 753 556");
print "$x\n";
$x = isABN("51 824 755 556");
print "$x\n";


sub isABN  # returns True if ABN, False if not an ABN
{
   my $abn = shift;
   $abn =~ tr/ //d;  #delete spaces
   
   return "False" unless (length $abn == 11);
   
   my @digits = split '',$abn;
   $digits[0] -= 1;
   my @weights = (10,1,3,5,7,9,11,13,15,17,19);
   
   my $sum =0;

   for my $i (0..10)
   {
      $sum = $sum + ($digits[$i] * $weights[$i]);
   }
   print "sum = $sum\n";  #for debugging...
   return ($sum % 89 == 0? "True": "False");
}

__END__
sum = 534
True
sum = 560
False
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