So now i have 2 scripts, one which is doing all work correctly but does NOT USE LINE BY LINE match the code is as follows

#!/usr/bin/perl -w

BEGIN {undef $/;}


# I TRIED 1ST my $match = "^module.*?$ARGV[2].*?([\\(;])"; 2ND my $mat
+ch = "\^module.*?$ARGV[2].*?([\\(;])"; 3RD my $match = "\\^module.*?$
+ARGV[2].*?([\\(;])"

#"(?<=module )$ARGV[2]"
my $match = "(?<=module )$ARGV[2].*?([\\(;])";
#print "$match";

my $filename = $ARGV[0];

open (INFILE, "<", $filename)  or  die "Failed to read file $filename 
+: $! \n";

$string =  <INFILE>;
close INFILE;



#I TRIED "$string =~ s/^$match/module $ARGV[1]$1/sg;";

$string =~ s/$match/$ARGV[1]$1/sg;
#print "$1";


open OUTFILE, ">$ARGV[0]" || die "Failed to create $ARGV[0]\n";
print OUTFILE ($string);
close OUTFILE;
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this will do well until in the comment// some1 writes our pattern. Now to come up with a new script which GOES LINE BY LINE courtesy @CountZero i have

use Modern::Perl;

my $filename = $ARGV[0];

die "Usage: modulenamechanger.pl filename newmodulename oldmodulename\
+n"
  unless @ARGV == 3;

open my $INFILE, "<", $filename or die "Failed to read file $filename 
+: $!";
say "Now parsing $filename";
my @file = <$INFILE>;
close $INFILE;

open my $OUTFILE, '>', $filename or die "Failed to create $filename";

my $match = qr "module\s*($ARGV[2])";
say "Matching: $match";

for my $line (@file) {
    if ( $line =~ m(^//) ) {
        print $OUTFILE $line;
        next;
    }
    $line =~ s/$match/module $ARGV[1]/;
    print $OUTFILE $line;
}
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But my problems with this are as follows: 1. NO modern perl 2.This code didnt work for me 3.I dont want to change the pattern regex my $match = "(?<=module )$ARGV[2].*?([\\(;])";(it may cause some other effect because im not 100% sure about its intent and why it was written in that way, so lets keep it that way only). 4. I didnt find any else portion in your code @zerocount 5.Also there is some problem in parsing, i have modified your code but still not getting desired o/p. Please see the below code, i cant find out whats wrong ,i tried various print statements as well to see but in vain

#!/usr/bin/perl -w

BEGIN {undef $/;}

#use Modern::Perl;

my $filename = $ARGV[0];

die "Usage: modulenamechanger.pl filename newmodulename oldmodulename\
+n"
  unless @ARGV == 3;

open my $INFILE, "<", $filename or die "Failed to read file $filename 
+: $!";
print "Now parsing $filename\n";
my @file = <$INFILE>;
close $INFILE;

open my $OUTFILE, '>', $filename or die "Failed to create $filename";

# WITHOUT ABANSAL CONCEPT my $match = qr "module\s*($ARGV[2])";
my $match = "(?<=module )$ARGV[2].*?([\\(;])";

print "Matching: $match\n";

for my $line (@file) {
    print "READING LINE $line \n";
    if ( $line =~ m(^//) ) {
        print "INSIDE IF\n";
        print $OUTFILE $line;
        next;
    }
    else 
    {

print "INSIDE ELSE \n BEFORE change:\t $line\n";
    $line =~ s/$match/moduleXXX $ARGV[1]/;
print "AFTER :\t $line\n";
    print $OUTFILE $line;
    next; 
    }
}
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The output is

my.pl f7 NEW OLD
Now parsing f7
Matching: (?<=module )OLD.*?([\(;])
READING LINE //Verilog HDL for "tt", "hh" "functional"
// if i write the word module OLD(Y, A, B ); here the script goofs up
`timescale 1ps/10fs


module OLD(Y, A, B );
    output Y;
    input A;
    input B;

endmodule

INSIDE IF
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