Oh ok. This is easy enough.

@foo = grep length(), @foo;

# is equivalent to but faster and clearer than

my @tmp;
for my $element_of_foo ( @foo )
{
    if ( length( $element_of_foo ) > 0 )
    {
        push @tmp, $element_of_foo;
    }
}
@foo = @tmp;
[download]

The line @foo = map qr/$_/, @foo; could be omitted from your program with no effect except of being slower. You'll see notes on qr// in perlop. The entire point is to pre-compile each regular expression so that when you do tests against it later perl won't have to do everything on the fly (and everytime you test a new file).