Most of your regex-fu is fine. The last, unsuccessful, one fails because it still matches any five words before looking for $WANT again. You could try a conditional second match to find the target string in the tail of the first match.

Another approach would be to split the text and grep for indexes of $WANT. Then produce slices of eleven elements around the positions you find and join with space.

my $text = "This is a test for the word blue more things blue and then
+ some very large text with a lot of things blue and done";
my @words = do {
    local $_ = $text;
    split;
};
my $WANT = qr/\bblue\b/i;
my @wants = grep { $words[$_] =~ $WANT } 0 .. $#words;
for (@wants) {
    my $first = ($_ - 5 < 0) ? 0 : $_ - 5;
    my $last  = ($_ + 5 > $#words) ? $#words : $_ + 5;
    print join( ' ', @words[$first .. $last]), $/;
}
__END__
a test for the word blue more things blue and then
the word blue more things blue and then some very large
with a lot of things blue and done
[download]