tHANKS Don Coyote, actually i tried what i suggested again... and fortunately its doing what i want, but i dont have any idea how its doing its job, specially the substitution part? we say it to replace $string with ARGV1$1.

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;

$string =~ s/$match/$ARGV[1]$1/sg;
print "$1";
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The output is just what i want i.e.

script.pl f5 NEW OLD
(?<=module )OLD.*?([\(;])
#> cat f5
//Verilog HDL for "tt", "hh" "functional"
// if i write the word module here the script goofs up
`timescale 1ps/10fs


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

endmodule
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please help me in understanding this