If the string may contain characters special to a regex that you want treated as regular characters to match, do /(\Q$string\E){3}/.

Make the match non-capturing, since you are unlikely to be using the captured result: /(?:\Q$string\E){3}/.

If the "3" is always the same and for some reason it isn't fast enough, try repeating it: /\Q$string$string$string\E/.

Save the regex in a variable and reuse it: my $regex = qr/\Q$string$string$string\E/; ... if ($line =~ $regex) ....

can any one suggest a better code to this??

#!/usr/bin/perl -w
my $string = "rid";
while (<>) {
if (/($string){3}/) {
print;
}
}
[download]

#!/usr/bin/perl -w

my $string = "rid";
while (<>) {
    if (/($string){3}/) {
        print;
    }
}
[download]

use strict;

my $string = 'ri\d+';
while (my $line = <>) {
    if ($line =~ m{ (?: \Q$string\E ){3} }xms) {
        print $line;
    }
}
[download]

the code above mentioned is not printing the matched content

Sorry, asdfghjkl, I confused you by changing the string. Try running it with ri\d+ri\d+ri\d+ for a match. That was meant to emphasize the use of \Q...\E.