This looks like a bug in perl. It seems that both the true and false part is evalueted wrongly, if there is '=' in the false case.
I've tried the following:

perl -e '$x="\nYo "; print ($x ? $x.="hello" : $x.="Bye");' => 'Yo hel
+loBye'
perl -e '$x="\nYo "; print ($x ?($x.="hello") : ($x.="Bye"));' => 'Yo 
+hello'
perl -e '$x="\nYo "; print ($x ? $x."hello" : $x."Bye");' => 'Yo hello
+'
perl -e '$x="\nYo "; print ($x ? $x."hello" : $x.="Bye");' => Can't mo
+dify concatenation (.) or string in concatenation (.) or string at -e
+ line 1, near ""Bye")"
Execution of -e aborted due to compilation errors.
perl -e '$x="\nYo "; print ($x ? $x."hello" : ($x.="Bye"));' => 'Yo he
+llo'
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Update:

Nope, not a bug, perl just doenst do what you expect ;( It's a precedence fault as masem points out.

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