#!/usr/bin/env perl

use warnings;
use strict;

# Usage: perl ./most-substrings.pl "R" ./example.txt

my $substr = shift @ARGV;

my %counts;
while (my $line = <>) {
  chomp $line;

  # Get a count of matching substrings:
  my @matches = $line =~ m/$substr/g;
  my $count = scalar @matches;

  $counts{$line} = $count
}

# Find the highest count of substring:
my $max = 0;
foreach my $count (values %counts) {
  $max = $count if $count > $max;
}

# For every line, check if its count is the highest we hit:
foreach my $line (keys %counts) {
  print "$line\n" if $counts{$line} == $max;
}