#!/usr/bin/perl
$_ = 'Patron @FirstName@ @LastName@ has filled out the @FormName@ form. Please send them an email at @EmailAddress@.';

s/@([a-zA-Z]+?)@/<b>\@$1\@<\/b>/g;

print $_ . "\n";
# Output: Patron <b>@FirstName@</b> <b>@LastName@</b> has filled out the <b>@FormName@</b> form. Please send them an email at <b>@EmailAddress@</b>.