$ echo '
Dear Monks,

I want to sort the attribute values inside <tag refid="5,8,7,3,2,1"> and remove the commas.

i.e. input: <tag refid="5,8,7,3,2,1"> some other text
output: <tag refid="1 2 3 5 7 8"> some other text

test.txt:
<tag refid="5,8,7,3,2,1"> some other text

' | perl -e'
$/ = ">";
while ( <> ) {
    s{(<tag refid=")([0-9,]+)(">)\z}{$1@{[ sort { $a <=> $b } $2 =~ /\d+/g ]}$3};
    print;
    }
'

Dear Monks,

I want to sort the attribute values inside <tag refid="1 2 3 5 7 8"> and remove the commas.

i.e. input: <tag refid="1 2 3 5 7 8"> some other text
output: <tag refid="1 2 3 5 7 8"> some other text

test.txt:
<tag refid="1 2 3 5 7 8"> some other text