$channel_data =~ s@\[@<lftsqbrk/>@g;
      $channel_data =~ s@\]@<rtsqbrk/>@g;
      $channel_data =~ s@\(@<lftbrk/>@g;
      $channel_data =~ s@\)@<rtbrk/>@g;
      $channel_data =~ s@\+@<plus/>@g;
      $channel_data =~ s@\?@<ques/>@g;
      $channel_data =~ s@\&apos;@<apos/>@g;
      $channel_data =~ s@\$@<dollar/>@g;

##</code><code>##

for ($channel_data) {
    s@\[@<lftsqbrk/>@g;
    s@\]@<rtsqbrk/>@g;
    s@\(@<lftbrk/>@g;
    s@\)@<rtbrk/>@g;
    s@\+@<plus/>@g;
    s@\?@<ques/>@g;
    s@\&apos;@<apos/>@g;
    s@\$@<dollar/>@g;
}

##</code><code>##

# 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;