#!/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;
}


##</code><code>##

sort -k2 -o file1 file1
sort -k2 -o file2 file2
join -j 2 -t '       ' file1 file2  # inserted a tab between the quotes with Ctrl-V