Update: sed doesn't have alternate delimiters, what was I thinking ...
You need to change your sed delimiters as they conflict with the slashes in $c and $b (or you could escape them) e.g

## warn if we get an abnormal exit status
system ("sed -e 's[$c][$b]g' $File1 > $Replace_html")
 == 0 or warn "sed had problems (exit val $?)\n";
[download]

Make sure you check your sed exit status in case of errors (check out system for details).
Also beware of using variables called $a or $b due to their automatic global nature (see. sort for more details).

Try this instead as escaping the slashes should do the trick

(my $search  = $c) =~ s[/](\\/)g;
(my $replace = $b) =~ s[/](\\/)g;
system("sed -e 's/$search/$replace/g' $File1 > $Replace_html")
  == 0 warn "sed had problems (exit val $?)\n";
[download]

You also might want to check that $File1 exists and is readable as $Replace_html won't be empty if sed exits normally.
HTH

_________
broquaint