Update: At first I considered your tr// statement to mean that you would be doing full security checking, but it has (rightly) been pointed out that there are huge security concerns here that it would be wrong to ignore. Read KM's reply below

1) In this line:

if (!(open (OUT, ">$_"))) { print "Error opening file $_ 
   for writing."; exit 0; }
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Try this:

if(!open(OUT, ">$_")){
    print "Error opening file $_ for writing: $!";
    # $! holds the error mesg
    exit 0;
}
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The more tradional way to write this is:

open(OUT, ">$_") or die "Can't open $_ for writing: $!";
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Since you are using this in a CGI program, it'd be a good idea to use perlfunc:warn or perlfunc:die too. (See the module CGI::Carp for http friendly die and warn functions)

But it doesn't seem to get to this point, so this isn't the source of the troubles. 2) some more information would be useful. You have the line:

print ">$_";
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Does it get printed? (Note that print statements might occur but not get printed if they are in the buffer when the program gets confused. Either use warn statements, or set $|=1, which will autoflush any statements in the buffer).