my $previous_line_matched = 0;
foreach my $line ( @lines ) {
    if ( $previous_line_matched ) {
        $line =~ s/^SY(?= )/  /;
    }
    if ( $line =~ s/^SY(?= )/CN/ ) {
        $previous_line_matched = 1;
        next;
    }
    $previous_line_matched = 0;
}