Many thanks for the additional information. As far as I can see I will have to experiment to get to the solution.
As a start I used the following three lines on a file with a ! in it.

$search-> {Text} = "\x{0021}";
$replace-> {Text} ="\x{f021}";
$exec_res = $search-> Execute({Replace => wdReplaceAll});
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This ‘worked’ and I was hoping to get a single wingding character. However I got three which the HexToolkit gave the following output following hex values EF 00 AC 20 A1 00 (these looked like an i with an umlaut, a Euro symbol and an i). How can I get just the one wingding that I want?