use strict;
use warnings;

my @FILES;

open $FILES[0], '<', 'file1' or die;
open $FILES[1], '<', 'file2' or die;

my @w = map scalar <$_>, @FILES;

my $empty = "blank line\n";
my %map = (
    -1 => [ 0 ],
    1  => [ 1 ],
    0  => [ 0, 1 ],
);

while (defined $w[0] && defined $w[1]) {
    my $cmp = $w[0] cmp $w[1];

    print $cmp ? $empty : $w[0];
    @w[ @{$map{$cmp}} ] = map scalar <$_>, @FILES[ @{$map{$cmp}} ];
}

# Leftover lines in either file?
print map $empty, map <$_>, @FILES;
[download]