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


chomp( my $input1 = <> );
chomp( my $input2 = <> );
my $output = 'output.txt';


open my $READ1, '<', $input1 or die "Cannot open $input1: $!.\n";
open my $READ2, '<', $input2 or die "Cannot open $input2: $!.\n";
open my $WRITE, '>', $output or die "Cannot open $output: $!.\n";

my $rewound = 0;
until ($rewound == 1 && eof $READ2
    || $rewound == 2 && eof $READ1
   ) {
    my $line = <$READ1> . <$READ2>;
    if (eof $READ1 && $rewound != 2) {
        seek $READ1, 0, 0;
        $rewound = 1;
    } elsif (eof $READ2 && $rewound != 1) {
        seek $READ2, 0, 0;
        $rewound = 2;
    }
    print {$WRITE} $line;
}
close $WRITE;
print "done\n";