use strict;
use warnings;

$_ =<<SQ;
<exm>This is example 1
<exm>This is example 2
<exm>This is example 3
<exm>This is example 4
SQ

s/<exm>/'<exm num="'.++$_.'">'/eg;

print;
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or just take the line number from the string:

s/<exm>(?=.+?example\s+(\d+))/<exm num="$1">/g;
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