Well, if < _$20_, you *need* this ##

##

use warnings;
use strict;

sub reg_fix {
    my $text = shift(@_);

    # I guessed what you wanted =equals= to indicate:
    my %tag= qw(  / em  * b  _ u  = strike  );

    my %open;
    @open{ qw( / * _ = ) }= ();

    my @tokens= $text =~ m{
      ( <[^<>]+>
      | [/*_=]
      | [^";
                my $nest;
                do {
                    $nest= pop @nested;
                    undef $open{$nest};
                } while(  $nest ne $token  );
                $token= "";
            }
        }
    }

    return join '', @tokens;
}

print reg_fix($_)   for  ;

__END__
go/to  /thx/  _silly *to_*
Well, if < _$20_, you *need* this

##

##

goto  thx/  silly *to*
Well, if < $20, you need this