If you're using a hash, you can probably speed things up by taking a hash slice with @banned_channels as a set of keys:

my @channels = split / /, '#perl #math #rpi #perlmonks';
my %where;
@where{@channels} = @channels;
@banned_in = map $_ ? $_ : (), @where{@banned};
[download]

Then again, I could be off my rocker.

$_="goto+F.print+chop;\n=yhpaj";F1:eval