change lines 5+6 to my %A; my @order=qw/horse lion mule tiger/;

Keywords to check are: soft references and auto-vivification: your strings 'a' to 'd' can be abused as soft references, which happens without use strict. With use strict however, you need a real hash-ref in the elements of %A instead - see perlreftut for more on references and soft references. Or omit the initialization of your hash-of-hashes %A to autovivify any undef values automagically as required (Autovivify definition or again: perlreftut).

and for your aligning your table layout, consider something like sprintf "%5s", $stringvariable