And a one-liner version is

   $exmno=0    unless s/<exm>/'<exm num="'.++$exmno.'">'/egs;
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Also note that in the OP, as here, the "++" goes before the "$exmno", otherwise you get

<exm num="0">This is example 1
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instead of

<exm num="1">This is example 1
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I assume you want the numbers to match.