use strict;
use warnings;
my $FILE = undef;
my $filename = 'C:\tmp\blah.txt';
open( $FILE, '<', $filename ) or die "Can't open $filename: $!";
while(<$FILE>) {
  ...
}
close $FILE or die "Close failed";
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Also, it looks like you are running Win32. /tmp/blah.txt is not a proper Win32 filename.

Update: Thanks to cdarke for pointing out Win32 filename subtleties, where /tmp/blah.txt indeed is a possibly filename.

/tmp/blah.txt is not a proper Win32 filename.

Works for me. It expects a directory called tmp on the current 'drive' (partition). mkdir tmp creates it. The problem with using backslashes as a directory separator is that you then MUST use single quotes around them, or 'escape' them. It is an easy mistake to put the filename inside double quotes. / is much safer, and works on Windoze happily.