use strict;

my %Counter;
open( K, "input" ) or die "input (keyword file): $!";
while ( <K> ) {
    chomp;
    $Counter{$k} = []; # initialize to (a reference to) an empty array
}
close K;

# concatenate the keywords with "|" to form a regular expression:
my $key_regex = join( '|', keys %Counter );

open( J, "solution.java" ) or die "solution.java: $!";
while ( <J> ) {
    while ( /\b($key_regex)\b/g ) {
        push @{$Counter{$1}}, $.;
    }
}
close J;

# now print a list of keyword hits:

for my $key ( sort keys %Counter ) {
    next unless scalar @{$Counter{$k}};
    print "$key found on lines @{$Counter{$k}}\n";
}