open my $FH, "/tmp/test.txt" || die "Failed to open /tmp/test.txt: $!\
+n";
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BTW: This statement from the OP has a bug. The relatively high precedence of the  || logical-or operator means that the truth value of the file name is logical-ored with the value returned by the die built-in (which, of course, never returns). Because a file name will almost always be true ('0' is the only one I can think of that would be false), the short-circuiting of the  || operator means that  die will almost never be evaluated and the statement will seem to 'work'. The  or logical operator has a sufficiently low precedence to make this really work. (See perlop.)

So the correct version of this idiom is something like
    open my $FH, "/tmp/test.txt" or die "Failed to open /tmp/test.txt: $!\n";
or (less elegantly IMHO)
    open(my $FH, "/tmp/test.txt") || die "Failed to open /tmp/test.txt: $!\n";

Update: This bug is hinted at by the fact that in the deparse of toolic's reply, the  die sub-expression has vanished! This is because the literal file name  "/tmp/test.txt" is always true, so the compiler softly and silently optimizes the  die away.