Why do you need to read the whole file into memory?

open my $FH, '<', $file or die $!;
while (<$FH>) {
    last if /^__END__\s/; # Update: maybe /\b__END__\b/ or /(?:^|\s)__
+END__(?:\s|$)/)
    print;
}
[download]

Also note that both the solutions incorrectly treat code like

print "
__END__
";
[download]