Why don't you believe Perl when it says that $trade_dt is not initialized?

Which one of these three lines is line number 42?

 my $tdate = $fields[0];
  print "$tdate\n";
my $trade_dt=UnixDate($tdate,"%Y-%m-%d");
#  my $trade_dt = DATE(\"$tdate\");
  print "$trade_dt\n";
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If you want to help us help you better, please post a complete, self-contained example of your program. Please do spend the effort and make it shorter than 20 lines.

As a debugging tip, I always do my debug print statements with some prefix and delimiters around the data:

...
print "TDATE: [$tdate]\n";
...
print "TRADE_DT: [$trade_dt]\n";
...
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That way, I can also see whether something is the empty string or did not print at all.

Update: My prediction is that you will see the following output:

TDATE: [10/14/2011]
TRADE_DT: []
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... and that would be because whatever unspecified date library you use does not raise an error when you pass it a malformed date. Your date input is in MM/DD/YYYY format, but you specify it as %Y-%m-%d format, and the conversion/deparsing fails.