So how is that any better than the following?

<SCRIPT language="PerlScript">
use Win32::Script qw/WScript/;
my $content = do {
    local $/;
    my $fh;
    open $fh, '<', 'a.txt' ? <$fh> : "Couldn't open a.txt: $!;"
};
$window->document->write($content);
</SCRIPT>
[download]

In fact I dare say it's worse - you were spawning an external process to read the file. There's sometimes something to be said for the brevity of qx{cat a.txt}, but you even gave up on that.

Makeshifts last the longest.