One problem: You have to tell Perl what you want to do with the filehandle you open. To read:

open my $INFO_FILTER, '<', $file_filter or die "Die: Could not open $f
+ile_filter: $!";
while ( <$INFO_FILTER> ) {
...
}
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(I give filehandles names in upper case for better readability.)

Also, use eq, not ==, when comparing strings.

Hope this helps!