while (<UNMATCHED>) { ... }

It's a little difficult for me to understand just what you expect from the posted code and what "failed to work," but one thing I suspect is that you may be expecting the value of  $_ set in the while-loop quoted above (a block read from a file) to be unchanged throughout the while-loop.

But while-loops do not "topicalize" the value of  $_ as for-loops do. I also see that there is a
    my $rtn = search ($fields[0], UNUSED);
statement in the quoted while-loop that seems to be searching a file with the handle UNUSED. If this search uses another
    while (<UNUSED>) { ... }
while-loop structure, the value of  $_ will be changed and will not be restored upon return to the scope of the posted code unless  $_ was properly local-ized within the search() function.

I think my advice would be to use lexical variables as the loop variable and in other cases within the block:

while (my $record = <UNMATCHED>) {
    my @fields = split(/:/, $record);
    ...
    }
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See my. Save your worries about  $_ for for-loops, map and grep built-ins, etc.