use HTML::TokeParser::Simple;
my $html =<<'END_HTML';
<table width="200">
    <tr>
        <td>foo</td>
    </tr>
</table>
END_HTML

my $parser = HTML::TokeParser::Simple->new(\$html);
my $new_html = '';
while (my $token = $parser->get_token) {
    $new_html .= $token->is_start_tag('table')
        ? new_width_attribute($token)
        : $token->as_is;
}
print $new_html;

sub new_width_attribute {
    my $token = shift;
    my ( $attr, $attrseq ) = ($token->return_attr, $token->return_attrseq);
    $attr->{ width } = '300';
    my $tag = '';
    foreach ( @$attrseq ) {
        $tag .= qq{$_="$attr->{$_}"};
    }
    $tag = "<table $tag>";
    return $tag;
}