"I have a file with words...I have a rule file... should be..."

OK, but was is the question? Doesn't it work? It isn't...? You don't know how to do it? Something else?

Regards, Karl

Update: Some inspiration can be found here: How do I post a question effectively?

G'day lakssreedhar,

You haven't actually asked a question - please take a look at: How do I post a question effectively?

For the scenario you describe, I'd be looking at a positive look-behind assertion - see perlre - Extended Patterns for details.

Here's an example:

$ perl -Mstrict -Mwarnings -E '
my @words = qw{door name dad naming};
for (@words) {
    s{(?<=na)m}{+d};
    say;
}
'
door
na+de
dad
na+ding
[download]

TWTOWTDI:

while(<FH>) {
    
my $inp_word4 = $_;

my $data = 'undef';
    
@inp_word=split(//,$inp_word4);
    
for(my $i=0;$i<@inp_word;$i++)
{

if (($inp_word[$i]=~m/(n)/i) && ($data eq 'undef') ){
$data =$1;
 next;
}
if (($inp_word[$i] =~ m/(a)/i) && ( $data eq 'n')){
$data = $data.$1;
next;
}
            
if (($inp_word[$i] =~ m/(m)/i) && ( $data eq 'na')){
$data = 'undef';
$inp_word[$i] = '+d';
next;
            
}
}

print  @inp_word  ;    
 
}
[download]

i have tested for the test cases "name domr fame naming namnam nanaming " and it's working.you can still make the code smaller.

in your code the if statement can never become true , your trying to match single character say 'n' with 'na'

If I understand your task description correctly, I think you just need a basic regex substitution:

while(<>) {
    s/(?<=na)m/+d/g;
    print;
}
[download]