If I were trying to do this, I'd start by matching constructs that could contain braces that are not significant (quotes, comments). And I'd match piecemeal rather than trying to build one big, complex regex.

    # load source code into $_
    while( m{['"{}]|//|/*|\bclass\s+}g ) {
        my $c = substr( $_, pos($_)-1, 1 );
        if( "'" eq $c ) {
            m{\G(?:[^\\']+|\\.)*'}gc
                or  fail( \$_, "Unclosed char" );
        } elsif( '"' eq $c ) {
            m{\G(?:[^\\"]+|\\.)*"}gc
                or  fail( \$_, "Unclosed string" );
        } elsif( '/' eq $c ) {
            m{\G[^\n]*}gc;
        } elsif( '*' eq $c ) {
            m{*/}gc
                or  fail( \$_, "Unclosed comment" );
        } elsif( '{' eq $c ) {
            $nest++;
        } elsif( '}' eq $c ) {
            $nest--;
        } elsif( 's' eq $c ) {
            my $p = pos();
            m{\G\w+}gc
                or  fail( \$_, "'class' not followed by ID" );
            my $name = substr( $_, $p, pos()-$p );
            if( m[\G\s*:\s*(?:\w+\s+)?(\w+)\s+{]gc ) {
                my $ancestor = $1;
                $nest++;
            } elsif( ! m[\s*{}gc ) {
                fail( \$_, "No { after 'class $name'" );
            }
        }
    }
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