Actually, the string was split according to the separator. But you put the separator into parentheses, which also put the separator into the result list. Maybe it is just the way that you are printing your results that confuses you?

#!perl -w
use strict;
use Data::Dumper;

my @a1 = split /abc/, "uno abc dos";
my @a2 = split /(abc)/, "uno abc dos";

print "a1: ", Dumper \@a1;

print "a2: ", Dumper \@a2;

__END__
a1: $VAR1 = [
          'uno ',
          ' dos'
        ];
a2: $VAR1 = [
          'uno ',
          'abc',
          ' dos'
        ];
[download]

These two variants of results are what you can get using a plain split. Others have already replied with ways you can modify those results to move, for example, the second element of the array into $1, but split does not set or modify the capture variables.