Sun	d
Mon	d
Tue	d
Wed	d
Thu	d
Fri	d
Sat	d
Jan	m
Feb	m
Mar	m
Apr	m
May	m
Jun	m
Jul	m
Aug	m
Sep	m
Oct	m
Nov	m
Dec	m

##</code><code>##

Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
January
February
March
April
May
June
July
August
September
October
November
December

##</code><code>##

#!/usr/bin/perl
use warnings;
use strict;

sub is_month {
    my ($line) = @_;
    return $line =~ /\tm$/ ? 1 : 0
}

open my $in_a, '<', 'a' or die "a: $!";
open my $in_b, '<', 'b' or die "b; $!";

open my $out_a, '>', 'a.out' or die "a.out: $!";
open my $out_b, '>', 'b.out' or die "b.out: $!";

while (my $line_a = <$in_a>) {
    my $line_b = <$in_b>;
    die "File b shorter!\n" unless defined $line_b;

    if (is_month($line_a)) {
        print {$out_a} $line_a;
        print {$out_b} $line_b;
    }
}
close $out_a;
close $out_b;

die "File a shorter!\n" unless eof $in_b;