Here's a couple of ways, but neither is necessarially the best way.

sub trim { 
    @_ = ($_) if not @_;  
    foreach my $s (@_) { 
        $s =~ s/^\s*//; 
        $s =~ s/\s*$//; 
        $s =~ s/\n\z//s; 
    } 
    return wantarray? @_ : $_[0]; 
}
[download]

Or

sub trim { 
    @_ = ($_) if not @_;  
    local $_; 
    foreach (@_) { 
        s/^\s*//; 
        s/\s*$//; 
        s/\n\z//s; 
    } 
    return wantarray? @_ : $_[0]; 
}
[download]

I think that the problem with your implementation was that you were stomping on the aliasing of $_ by re-using the global $_ implicitly in your foreach loop. Using either a my'd var or localising $_ after the assignment to @_ but before the foreach loop seems to fix the problem, though I have a gut feel that there is probably a better way.