($foo,undef) = split( /\./, $bar );
That way I explicitly throw away the rest of the list instead of discarding it implicitly.

You're not explicitly throwing away the rest of the list... you're just explicitly ignoring the second element returned and then non-explicitly ignoring the rest of it. Using undef to ignore returned elements is only useful if you're going to capture something after the undef'ed element (ie: ($foo, undef, $bar) = split(/\./, $baz); or (undef, $foo) = split(/\./, $bar);).

TIMTOWTDI of course:

# this:
($foo, undef, $bar) = split(/\./, $baz);

# can also be done as:
($foo, $bar) = (split(/\./, $baz))[0,2];


# and this:
(undef, $foo) = split(/\./, $bar);

# can also be done as:
($foo) = (split(/\./, $bar))[1];
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Oooooohhhh, but that builds up an entire list for the slicing and can damage performance horrendously:

#!c:/perl/bin/perl -w
$|++;

use strict;
use Benchmark 'timethese';

# make it so we split() into many many elements
our $baz = join '', "yes.no.maybe.so." x 5000;

timethese(2000, {
   undef => q{
      ($foo, undef, $bar) = split(/\./, $baz);
   },

   splice => q{
      ($foo, $bar) = (split(/\./, $baz))[0,2];
   }
} );

__END__
Benchmark: timing 2000 iterations of splice, undef...

     splice: 101 wallclock secs (93.86 usr +  0.39 sys = 94.25 CPU) @ 
+21.22/s (n=
2000)
     undef:  0 wallclock secs ( 0.14 usr +  0.00 sys =  0.14 CPU) @ 14
+184.40/s (
n=2000)
            (warning: too few iterations for a reliable count)
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