Another WTDI would be to use sprintf, if only to make the code more readable:

my $fmt = '<section id="ch%02dfm" label="" xreflabel="" role="fm">';
# ..later..
$_ =~ s/<CT>/sprintf $fmt, $acno/ge;
[download]

BTW, the default target for the substitution is $_ so you could actually shorten the assignment to just

s/<CT>/sprintf $fmt, $acno/ge;
[download]