@a2 = split /(abc)/, "uno abc dos";
... some use of $1 ...

##</code><code>##

#! /usr/bin/perl

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

print "@a1\n"; # prints "uno   dos", as expeted
print "@a2\n"; # prints "uno  abc  dos" !!! :(