in reply to Foreach Loops

I'm surprised no one has suggested using map for this purpose. That way, it is easy to track the previous element .. 
my $prev;
map { print qq($prev,$_ \n); $prev=$_} qw(uno dos tres);

##Output
,uno
uno,dos
dos,tres

[download]
Update: I just noticed that holli's solution above is functionally identical.

    ..."I don't know what the facts are but somebody's certainly going to sit down with him and find out what he knows that they may not know, and make sure he knows what they know that he may not know, and that's a good thing. I think it's a very constructive exchange," --Donald Rumsfeld

In Section Seekers of Perl Wisdom