You can use $chr(ascii-value), in your case this is: $chr(124). There are charmaps for reference, and you can quicky find out the ascii of some character with mIRC itself: use //echo -a $asc(char), e.g.: //echo -a $asc(|)
Alternatively, you can use $eval(char,0) - in your case $eval(|,0) in your code.