You're missing the closing backtick and semicolon.

It is pretty easy to emulate system grep -l in perl,

my @arr = grep {
    local $/;
    open my $fh, '<', $_ or die $!;
    my $text = <$fh>;
    $text =~ /<value>IsFrequentlyUsed<\/value>/;
} glob '*';
[download]

If your files are large, you may wish to read them one line at a time, instead.