my $string = 'MB 1 UL L F UT 100 C 20CT'; 
$string =~ s/^((?:\w+\s+){7})(?!(?:BX|P)\s+)/$1P /; #inserting P attribute 
my @attrs = split(/s+/, $string); 
$attrs[7] = '' if $attrs[7] eq 'P'; # processing P attribute

open (INFILE, "< a.txt") || die "can not read: $!"; 
open (OUTFILE, "> out.csv") || die "can not write: $!"; 
 
while ( my $string = <INFILE> ){ 
  $string =~ s/^\W+//; # removing garabge in line start 
  $string =~ s/\W+$//; # removing \n\r etc at the end of line 
 
  # inserting P attribute via "replace" )) 
  # 'MB 1 UL L F UT 100 C 20CT' -> 'MB 1 UL L F UT 100 P C 20CT' 
  $string =~ s/^((?:\w+\s+){7})(?!(?:BX|P)\s+)/$1P /;  
 
  # the following makes array of strings like 
  # ('MB','1','UL','L','F','UT','100','P','C','20CT') 
  my @attrs = split(/s+/, $str);  
 
  # if 8th (index is 7 because started from 0) element is 'P' 
  # making it blank according to the requirements 
  $attrs[7] = '' if $attrs[7] eq 'P'; 
 
  # printing into CSV data file (readable by excel etc) 
  print OUTFILE join(',', @attrs)."\n"; 
} 
close INFILE; 
close OUTFILE;

my @input = split( /\s+/, $string ); 
my @attrs = (); 
my $index = 0; # the position 
while ( @input ) { 
	$index++; 
	my $current = shift @input; # reading next input 
 
	if( $index <= 2 ){  
		# just copy first two walues 
		push @attrs, $current;  
		next; # skip other checking and go to next value read 
	} 
 
	if( $index == 3 ){ 
		if( $current =~ /^(?:UL|L|DUL|DLX|M)$/ ){  
			# process eligible value 
			push @attrs, $current;  
			next; 
		} 
		# the current element is not eligible 
		# to be 3rd, so setting 3rd element to 'R' 
		# and treat current value as next element (4th) 
		push @attrs, 'R'; 
		$index++;  
	} 
 
	if( $index == 4 ){ 
		# same way 
		if( $current =~ /^(?:M|R)$/ ){  
			push @attrs, $current;  
			next; 
		} 
		push @attrs, 'R'; 
		$index++; 
	} 
 
	if( $index == 5 ){ 
		# same way 
		if( $current =~ /^(?:F|NF)$/ ){  
			push @attrs, $current;  
			next; 
		} 
		push @attrs, 'NF'; 
		$index++; 
	} 
 
	# insert processing for 6-7 here 
 
	if( $index == 8 ){ 
		# following is example and could be used for any index 
		# checking condition to skip the attribute 
		# with additional check for next (further) value 
		if( $current =~ /^BL$/ && $input[0] =~ /(?:BX|P)/ ){ 
			# skipping BL and reading next value for next if() 
			$current = shift @input; 
		} 
		if( $current =~ /^(?:BX|P)$/ ){  
			# push blank string if 'P' 
			push @attrs, ( $current ne 'P' ) ? $current : '';  
			next; 
		} 
		push @attrs, ''; # default is blank string 
		$index++; 
	} 
	# insert processor for 9-10 here 
}

while ( my $string = <INFILE> ){  
  $string =~ s/^\W+//; # removing garabge in line start  
  $string =~ s/\W+$//; # removing \n\r etc at the end of line  
  
  my @input = split( /\s+/, $string );  
  my @attrs = ();  
  my $index = 0; # the position  
  while ( @input ) {  
 
    # place conditions by index here 
   
  } 
  # printing into CSV data file (readable by excel etc)  
  print OUTFILE join(',', @attrs)."\n";  
}

if( $input[0] =~ /^(?:BX|P)$/ || scalar @input == 2 ){ 
  # assign new value to $current 
  # it means the previous value will not be  
  # taken into account anymore = ignored 
  $current = shift @input; 
}