my $test = 'Patron @FirstName@ @LastName@ has filled out the @FormName@ form. Please send them an email at 

@EmailAddress@.';

$test =~ s/(?<=@)		# starts with @ (but don't grab @)
	   ([a-zA-Z]*)		# string of letters
           (?=@)		# ends with @ (but don't grab @)
	  /<b>$1<\/b>/xg;	# put $1 back with bold tags around it. @ are preserved.

print $test;