The code fragment appears to be wrong.

• If you're reading from <STDIN> then you'll need to chomp() the input otherwise the open will more than likely fail. You may think you're asking it to read file "myfile.txt", but really it'll try to open "myfile.txt\n".
• Why are you array dereferencing $filename in the open()? It's a scalar value, not an array so what you're doing is reading the file name into a variable called $filename but attempting to read it from an array called @filename (which are different things).

Try this:

my $filename = <STDIN>;
chomp($filename);
open (FH,$filename) or die "Unable to open file: $!";
my @Lines = <FH>;
close FH;
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You should also check your return values when attempting to open files otherwise it could silently fail and then fall over later when the code you expect isn't there... The $! is very handy for rooting out why something won't open.

I didn't manage to get it to throw the error you came up with so perhaps where Perl is reporting isn't actually the root of the problem (which is fairly common with most programming languages!).