In the spirit of TMTOWTDITDUG, here's my idea:

#!/usr/bin/perl -w
use strict;

my ($fh1, $fh2);
open $fh1, "file1.dat" or die "I no read file 1.\n";
open $fh2, "file2.dat" or die "I no read file 2.\n";

my $mainfh = $fh1;
while (<$mainfh>) {
   print;
   $mainfh = ($mainfh == $fh1) ? $fh2 : $fh1;
}
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file1.dat:

I'm
sure
this
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file2.dat:

pretty
that
works.
[download]

And finally, output:

I'm
pretty
sure
that
this
works.
[download]

His Royal Cheeziness