# Outside the loop

my %subst = (
    '[' => '<lftsqbrk/>',
    ']' => '<rtsqbrk/>',
    '(' => '<lftbrk/>',
    ')' => '<rtbrk/>',
    '+' => '<plus/>',
    '?' => '<ques/>',
    '&apos;' => '<apos/>',
    '$' => '<dollar/>',
);

my $re=join '|', map quotemeta, keys %subst;

# ...
# Inside the loop

$channel_data =~ s/($re)/$subst{$1}/g;