In addition to Eilys hint, try

print $filename, "\n";
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Note that, if you have your complete filename in a variable, like in your OP (and as you should, as it eases debugging and error messages), quoting it in the open statement is superfluous.

Besides, most Windows functions and many programs accept forward slashes, so you can write

my $filename="$mydir/file_detail.txt";
open (FILE, "<", $filename);
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Update: corrected quotes - d'oh! Thanks AnomalousMonk!