use strict;
use warnings;

my %content;
for my $animal ('dog', 'cat') {
    open my $fh, '<', "/tmp/$animal.txt" or die "Couldn't open $animal
+: $!";
    while ( my $line = <$fh> ) {
        ++$content{$animal}{$line};
    }
}

for my $animal ('horse', 'lama', 'camel', 'tiger') {
    open my $fh, '<', "/tmp/$animal.txt" or die "Couldn't open $animal
+: $!";
    while ( my $line = <$fh> ) {
        for my $other ('cat', 'dog') {
            print "$other: $line" if $content{$other}{$line};
        }
    }
}
[download]

Is this a toy exercise for learning, or have you some non-trivial application? If you have a non-trivial application you really ought to tell us about it because the implementation details depend a great deal on the nature of the application. Things like size of files and length of match strings make a huge difference to how to efficiently implement the search.

perl -lne '%ln;BEGIN{open $f,shift;map{chomp;$ln{$_}++}<$f>}print qq($
+ARGV line\t$.\t[$_]) if exists $ln{$_};close ARGV if eof' 
           dog.txt cat.txt other.txt  

cat.txt line    2       [pretty woman]
other.txt line  1       [pretty woman]
[download]

Hoping everyone had a great New Year and thanks for the replies. I started trying some more tactics to find out how to compare columns and lines. I am trying it as an exercise. There are actually not 50 files, but I meant it could be more than 2. Here are actual files I created: first.txt

/vol/cat,feline
/vol/dog,canine
/vol/cat,feline
/vol/cat,feline
/vol/amphibian,FROG
/vol/amphibian,FROG
[download]

second.txt

9,/vol/elephant,fourfeet
1999,/vol/dolphin,fish
10,/vol/cat,feline
1111,/vol/goldfish,fish
2222,/vol/spider,arachnid
5555,/vol/camel,dromedary
3333,/vol/wolf,canine
[download]

I am trying to do the following: 1. select the /vol/cat,feline as the element common in the array - this will require that the first column be excluded. 2. If the /vol/cat,feline is found (if an element is found in common to the array, print the ID # - for example #10. Here is what I did so far:

use strict;

sub get_animal {
  open my $FILE, '<', shift or die $!;
  return map {chop; $_ => $_} <$FILE>;
}

my %a = get_animal '/tmp/first.txt';
my %b = get_animal '/tmp/second.txt';

{
  
  print "$_\n" for grep {$_} @a{keys %b};
}
[download]

It works if the column with the numbers in it is deleted from second.txt. I don't know how to make it compare the first and second columns from first file with the second and third columns from the second file. After that, it needs to return the ID # when it finds a match. Any ideas?

Hello, i cannot fully understand your requirements, given the two example files; can you rephrase?

Some observations:
• where is use warnings;?
• do not use uppercase variables names
• also rembember to close your filehandles anyway: it is safer.
• chop is not chomp
• Avoid a or b as variable name: the scalar form are special variables and even if the hash is not, avoid it anyway. Instead choos meaningfull variables names
• when learning or debugging i think is preferable to write down plain syntaxes: you have a superfluous bare block: why? the syntax inside it is not so begenner's one. How can inpsect it without a place where insert the basic debugging tool aka print?

#{
#  print "$_\n" for grep {$_} @a{keys %b};
#}
#
# should be something like (untested..)

foreach my $bkey (keys %b) {
    warn "key not defined" unless $bkey; ## what is the purpose of you
+r "grep {$_}"???
    if ($a{$bkey}){print "FOUND: [$bkey] in the hash \%a\n"}
    else{print "NOT found key [$bkey] in the hash \%a\n"}
}
[download]


L*


There are no rules, there are no thumbs..
Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.

Perhaps you should rethink your actual needs.