rule "mvel equal sign bug"
dialect "mvel"
when
$d: Dog(name == "Bertie")
then
modify ($d)

{name = "Bertie = Boy"};
end

Note the string literal for the new value for name contains an equal sign (=). That causes an error. Apparently, the modify macro or the MVEL parser is choking on the equal sign.

java.lang.Error: org.drools.rule.InvalidRulePackage: Unable to build expression for 'consequence': [Error: unterminated literal]
[Near : {... h ($d) {name = "Bertie = Boy"}

....}]
^
[Line: 1, Column: 43] ' modify ($d)

{name = "Bertie = Boy"}

;
' : [Rule name='mvel equal sign bug']

If I remove the "=" from inside the string literal (substituting "is a"), it works as expected.

rule "mvel equal sign bug"
dialect "mvel"
when
$d: Dog(name == "Bertie")
then
modify ($d)

{name = "Bertie is a Boy"}

;
end