$data{STATEMENT} =~ s/\b\d+\b/$data{CATNUMBER}/;
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would be one way, but it would only work if you don't have a string of the form "There is 1 grey and 4 ginger cats". If you do, a bit more sophistication is involved.

If (a big condition, I realize) you've got control over how the file is set up, you could consider some distinct markers for the values you want to replace.

You've really got to be a bit more forthcoming with your requirements and expectations. Giving one case, which may very well be special, is really not a good way to ask a question