my $mon_idx = 0;

for my $fname ( qw(jan.dat feb.dat mar.dat apr.dat may.dat) ) {
    
    open my $fh, "<", $fname or die "Couldn't open '$fname': $!";
    
    while (<$fh>) {
        my ($acc, $freq, $perc) = split /\t/;
        $mutperc{$acc}[$mon_idx] = $perc;
    }
    $mon_idx++;
}

for my $acc (sort keys %mutperc) {
    print $acc;
    for my $mon_idx (0..4) {
        my $perc = $mutperc{$acc}[$mon_idx];
        $perc = 0 unless defined $perc;
        print "\t$perc";
    }
    print "\n";
}