This is explained in the Perl documentation.
Because of Perl operator precedence the statement is being parsed as
($condition ? $a= 2 : $a ) = 3 ;
Because the ?: operator produces an assignable result, 3 is assigned to the result of the condition.
When $condition is true this means ($a=2)=3 giving $a=3
When $condition is false this means ($a)=3 giving $a=3
The correct way to write this is
$a = ( $condition ? 2 : 3 );
print $a;
We got bitten by this at work, so I am posting here hoping others will find it useful.