You say that you tested your data, but this line:

$account->{transactions} = \{get_transactions($account->{transactions_
+id})};
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... feels suspicious to me. I think you're double-referencing things here, given that get_transactions already returns a hash reference.

I suggest you compare the Data::Dumper output of your data structures and pay very close attention to whether you get a hash reference or a reference to a hash reference:

>perl -MData::Dumper -wle "warn Dumper \{ foo => bar }; warn Dumper { 
+foo => bar }"
Unquoted string "bar" may clash with future reserved word at -e line 1
+.
Unquoted string "bar" may clash with future reserved word at -e line 1
+.
$VAR1 = \{
            'foo' => 'bar'
          };
$VAR1 = {
          'foo' => 'bar'
        };
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Also, "I have an ISA problem" is not a very good error description. Maybe you want to tell us the exact error message that HTML::Template gives you?