Are the files in the same order? ....seems like not, and that's somewhat relevant to the solution. A set of ordered lists where each line is a direct match for the same line number in the other file would lead to an O(n) solution. If the files don't have a "line number for line number" relationship, then the solution becomes computationally more complex. A hash could be used, but the computational complexity does increase.

By the way; this problem does seem like something that may have been approached in some form previously. And in fact there is a module Lingua::EN::Conjugate. But it addresses the general need for conjugation, not the specific task of matching up verb tenses and persons.

Here's one solution that could work if the word-sets can fit into memory all at once. With the magic of references and the uniqueness as well as associativity of hash keys, the solution just sort of falls into place with a single hash.

use strict;
use warnings;
use v5.14;
use autodie;

my %word_sets;

while( <> ) {
    chomp;
    my ( $conjugation, $infinitive ) = split /\s*\|\s*/;
    push @{ $word_sets{ $infinitive }, $conjugation;
}

open my $outfh, '>', 'combined_conjugations.txt';
while( my( $infinitive, $conjugations ) = each %word_sets ) {
    next unless @{$conjugations} > 1; # Skip items that didn't appear 
+twice.
    say $outfh join( ' | ', $infinitive, @{ $conjugations } );
}
close $outfh;
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By your problem description it looks like we can assume that in each file the alternate conjugation comes before the infinitive, whereas your solution file would list the infinitive first on each line, followed by the alternate conjugations. I took care to preserve what seemed to be your intent in this respect. What I didn't preserve, however, is any notion of line ordering. You probably do want some form of sorting, but it wasn't apparent in your question. If you do need the list to be sorted, or to preserve some original order, you'll have to modify the solution provided.

Since you only want the ones that are contained in both files, you only need one hash, containing a hash or an array (your choice, really) for each infinitive found. Then go through the hash and output all those that have both parts filled.

#!/usr/bin/perl

my %k; # hash to hold infinitives and their other parts

process( 'thirdperson', 'file1');
process( 'past', 'file2');

for (keys %k){
    if( $k{$_}{thirdperson} and $k{$_}{past} ){
        print "$_\t$k{$_}{thirdperson}\t$k{$_}{past}\n";
    }
}

sub process {
    my $part = shift;
    my $filename = shift;
    open my $fn, '<', $filename or die $!;
    while(<$fn>){
        chomp;
        my($other, $infinitive) = split /\t/;
        $k{$infinitive}{$part} = $other;
    }
    close $fn;
}
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I used a hash of hashes, because I thought that would make what I was doing clearer. Each hash within %k is only printed if it contains values for both keys 'thirdperson' and 'past'. But you could also use a two-element array, as long as you keep track of which form of the verb goes in which numbered element.

Edited to add: The unix command 'join' also does this, printing lines from multiple files that share a common field. Both files have to be sorted first by the field being compared, though.

sort -k2 -o file1 file1
sort -k2 -o file2 file2
join -j 2 -t '       ' file1 file2  # inserted a tab between the quote
+s with Ctrl-V
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1 open file 1 and load its contents to a hash

2 open and load the file 2 to another hash

so you want to compare 2 hashes, see more examples in supersearch

If your two input files have just the vertical-bar character as the column delimiter (without spaces), then this command line:

cmpcol -i -lb '|' -d '|' file1:2 file2:2
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goes|go|went|go
sees|see|saw|see
eats|eat|ate|eat
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Then all you have to do is re-arrange the columns to suit your taste:

cmpcol -i -lb '|' -d '|' file1:2 file2:2 | perl -F'\|' -lape '$_=join 
+"|",@F[1,0,2]'
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