srikdatta:

You were close, but you're getting the error because you haven't done anything to put a value in $_. It appears like you're wanting to scan through a file, so you'd do something like this:

#!/tool/pandora64/bin/perl5.8.8 -w

use strict;
use warnings;

my @verilogfiles = glob '*.verilog';
my ($vf, $of, $i, $j);

print "Define the path for sdf_annotate.v: ";

my $path = <STDIN>;

chomp($path);

#For verilog files
foreach my $b (@verilogfiles)
{
       open $vf, '>>', $b or die "Could not open file '@verilogfiles' 
+$!";
       while (<$vf>) 
       {
           if($_ =~ /module entity\;/)
           {       
               print $vf "\`include \"$path\"\n";
           }
       }
}
close ($vf);
[download]

The while (<$vf>) { loop will load a line from $vf into $_ and execute the loop body, and repeat until there are no more lines in the file.

...roboticus

When your only tool is a hammer, all problems look like your thumb.