use Text::CSV_XS;
use IO::File;

my $csv = new Text::CSV_XS({sep_char=>"\x1E", escape_char=>"\x10"});

my $io  = new IO::File('< flat_file.txt');

until ($io->eof) {
   my $line = $io->getline();
   $line =~ s/_UNSC/\x10/g;
   $line =~ s/(?<!\x10) _ /\x1E/g;

   my $column = $csv->parse($line);
   ## .. do something with the ArrayRef $column .. ##
}
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This code reads each line (I assume each line is a record), converts escape characters to \x10 (ASCII 'Data link escape') and your record-separator string to \x1E (ASCII 'Record separator') in memory, then passes that much-cleaner line to the Text::CSV_XS object for parsing.

This way, your manager won't see it as file conversion, because you're merely caching a simplified version in memory from which to work. You can reverse the substitution process if you need to save a new file in the same format.

Of course, once you've got code like this working, it would be trivial to write the file back out in a newer format whenever your boss changes his/her mind.