The strangeness seems to be in the diamond operator. If I use a sub instead, I get a warning:

#!/usr/bin/perl -w

open FOO, "< /dev/null"
    or die "No foo: $!\n";
#scalar(<FOO>) =~ /bar/
(rf() =~ /bar/)
    and print "bar\n";

sub rf
{
  return scalar(<FOO>);
}
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