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!

In reply to Re^3: While is not working when using variable by soonix
in thread While is not working when using variable by parthodas

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