use 5.010;

...

my $pre_b    = qr{ ... }xms;
my $pre_weis = qr{ ... }xms;

my $b_was_seen;

while (my $line = <$input_filehandle>) {

    # check for trigger condition/substitution pattern sequence.
    # ASSUME: 'b' and 'Weis' cannot both occur in same line.
    if ($line =~ m{ \A $pre_b b }xms) {
        # flag 'b' was seen in this line.
        $b_was_seen = 1; 
        }
    elsif ($b_was_seen) {
        # attempt substitution if 'b' seen in previous line.
        $line =~ s{ \A $pre_weis \K Weis }{Schwarz}xms;
        $b_was_seen = 0;
        }
        
    # write each (possibly altered) line to output file.
    print $output_filehandle $line;
    
    }