In double quotes, backslashed are special. Use single quotes:

open fh, 'C:\windows\file.txt';
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Note that using lowercase barewords for filehandles begs for troubles. It is safer to use lexical filehandles, 3-argument form of open, and also check the return value of open in case the opening did not succeed:

open my $FH, '<', 'C:\windows\file.txt' or die $!;
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See open for details.