while ($fol_wzg_cel =~ /<B6>/) {$fol_wzg_cel =~ s/<B6>/%B6/;}

# as:

$fol_wzg_cel =~ s/<B6>/%B6/g;

##</code><code>##

$fol_wzg_cel =~ /<(B6|B1|F3|A6|AC|D3)>/%$1/g;