use strict;
use warnings;

my %result;

open my $A, "<", "file1" or die "could not open file1 $!";
while (<$A>) {
    chomp;
    my $key = (split /\t/, $_)[0];
    $result{$key} = 1;
}
close $A;
open my $B, "<", "file2" or die "could not open file2 $!";
while (<$B>) {
	chomp;
    my $key = (split /\t/, $_)[0];
    $result{$key}++;
}
close $B;
open my $C, "<", "file3" or die "could not open file3 $!";
while (<$C>) {
    chomp;
    my $key = (split /\t/, $_)[0];
    if ($result{$key} == 2) { # this key has been seen in both previous files
    	print "Line with $key is present in all three files\n";
    }
}	
close $C;