XXX="/blah1/blah2/blah3/1234567890;arg=AAA123BBB456CCC"
#         alphanumeric: ^^^^^^^^^^
#                       optional: ^^^^^^^^^^^^^^^^^^^^
#                   fixed string: ^^^^^
#                        alphanumeric: ^^^^^^^^^^^^^^^
echo "$XXX" | perl -n -e 'if (m[^/.+/([^/\?\s]+)(?:;arg=)?]) {print "$1\n";}'

##</code><code>##

echo "$XXX" | perl -n -e 'if (m[^/.+/([^/\?\s;]+)(?:;arg=)?]) {print "$1\n";}'
                                             ^