The reason your map gives you the empty string in spite of your if is that it always gives you the last expression evaluated, much like do or a sub. Consider this timeless classic (from Evil Interview Questions):

sub baz { return 11 unless shift }

print 'baz(5): ', baz(5), "\n";
print 'baz(0): ', baz(0), "\n";
__END__
baz(5): 5
baz(0): 11
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If you want to take something out of a list processed with map, have the block explicitly return an empty list.

my @a = ( qw (onexxx txwxo txhrexe xfourx xxx five) );
my @b = map {
        $_ =~ s/x//g;
        $_ ? $_ : ();
        } @a;
print Dumper(\@b);
__END__
$VAR1 = [
          'one',
          'two',
          'three',
          'four',
          'five'
        ];
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As a side note, you should know that modification of $_ inside map modifies the original list element. Your code (and mine) leaves @a looking like this:

$VAR1 = [
          'one',
          'two',
          'three',
          'four',
          '',
          'five'
        ];
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You can use Benchmark to try to find the most efficient way to do something, but for something like this I think you'd have to have a pretty long list before it would make a significant difference. It's generally best to use the most easily understood code until profiling tells you that something is worth optimizing.