# Create a hash of codes for substitution
my %directSubs = (
                  qr/\[h(\d)\]/  => '<h$1>',
                  qr/\[\/h(\d)\]/=> '</h$1>'
                  );

# Substitute the codes
while(($a,$b) = each %directSubs){
   $line =~ s/$a/$b/gi;
}