$a =~ s/(?<=\|)|^xx(?=(\||$))/x x/g;

# and here is a way using the zero width boundary assertion \b
# that will match | ^ or $ but also matches any non aphlanumeric 
# so would potentially fail on '|xx,xx|' type strings
# if they exist in practice as your real data is no doubt
# not really xx.....
$a =~ s/\bxx\b/x x/g;