You can't as you have it. However since you're loading the code with do you can write any valid Perl and it'll work fine.

$data{ 'CATNUMBER' } = 6;
$data{ 'STATEMENT'  } = "THERE EXIST $data{ 'CATNUMBER' } CATS";
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(Minor style point: note that the quotes on the keys aren't strictly needed since you're using the fat comma => operator, nor are they needed in my sample; however I've started to use them myself after getting into the habit in Ruby where you can't omit them :)