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my $a = "xx|xx|xx|xxxx|xx|xxx|xx|xx|xx";
$a =~ s/(?<=\|)xx(?=(\||$))/x x/g;
print $a;
__DATA__
xx|x x|x x|xxxx|x x|xxx|x x|x x|x x

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

...
# if they exist in practice as your real data is no doubt
# not really xx.....
$a =~ s/\bxx\b/x x/g;