If you're talking about this section of the code:

$_ = $line;
s/$ARGV[0]/($ARGV[0])/g;
print $line;

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It's because $line and $_ are two different variables. your s/// is changing $_ there, so that's what you'd need to print out if you wanted to see the change.

Note: I'd change a lot of things about your script, but starting with adding use strict and use warnings to the top. Here is a cleaned up version with strictures in place:

use strict;
use warnings;

my $string = shift;

my $file = 'electricity.txt';
open my $fh, '<', $file or die $!

my $counter = 0;
while (<$fh>) {
    if (s/\Q$string\E/($string)/g) {
        $counter++;
        print "$counter ";
    }
    print;
}

close $fh;

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In reply to Re^5: The $_ Special Variable by wind
in thread The $_ Special Variable by chuloon

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