I may be missing some subtleties here, but if you want a no-arg call to operate on $_ (handy shortcut) and you don't want a call with an empty array arg to operation on $_ by mistake (nasty unexpected side-effect), the way to write the sub would (I think) be like this:

sub trim
{
    my @args = ( @_ ) ? @_ : $_;
    for ( @args ) {
        s/\A\s+//;
        s/\s+\z//;
    }
    return wantarray ? @args : $args[0];
}
[download]

If you pass an empty array to that verion, it will not modify $_.

Update -- test suite:

#!/usr/bin/perl

use strict;

sub trim
{
    my @args = ( @_ ) ? @_ : $_;
    for ( @args ) {
        s/\A\s+//;
        s/\s+\z//;
    }
    return wantarray ? @args : $args[0];
}

$_ = " test  ";
print "test 1: ==" . trim . "==$_==\n";

$_ = " 1234  ";
my @b = ();
trim( @b );
print "test 2: ==". $_ . "==@b==\n";

my @c = ( "  a ", " b  ", " c " );
my $d = join "", trim( @c );
print "test 3: ==". $_ . "==" . $d . "==@c==\n";

__OUTPUT__
test 1: ==test== test  ==
test 2: == 1234  ====
test 3: == 1234  ==abc==  a   b    c ==
[download]

Having updated "test 1" to print out both the return value of trim and $_, I see that $_ is never modified, which I gather is not what you want. Sorry -- I get your point now, and I don't have an answer.