my $full_line = '';
while ( my $line = <FH> )
{
    chomp $line;
    next if substr( $line, 0, 2 ) eq '//';
    $full_line .= " $line"; # space wanted? your call!
    next unless substr( $line, -1, 1 ) eq ';';

    #do something with $full_line
    print "$full_line\n";

    $full_line = '';
}

##</code><code>##

    $full_line .= $full_line eq '' ? $line : " $line";

##</code><code>##

some code;
more code;
some very long long long code;