It's impossible for nothing to go into $_ if the code that uses $_ gets executed. $_ is being aliased to each element of @trans_details in turn, so $_ will contains whatever they contain. If the value of $_ doesn't change, it's because the value of the elements are the same.

In other words, the bug is elsewhere.

Update: As kennethk discovered, there is a problem with the code you posted. The problem isn't with $_, though. I wonder how you came to the conclusion that it was.