What I’d do differently now: at least this:

sub trim2 {
    return trim2 $_ if not @_;
    return map { s/^\s+//; s/\s+$//; $_ } my @c = @_
        if defined wantarray;
    for ( @_ ) { s/^\s+//, s/\s+$// }
}
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And someone who can be bothered should benchmark the following variants:

sub trim3a {
    return trim3a $_ if not @_;

    s/^\s+//, s/\s+$// for
                wantarray ? my @c = @_     :
        defined wantarray ? my $c = $_[-1] :
                                    @_;

    @c ? @c : $c ? $c : ();
}

sub trim3b {
    return trim3b $_ if not @_;

    my $w = wantarray;
    s/^\s+//, s/\s+$// for
                $w ? my @c = @_     :
        defined $w ? my $c = $_[-1] :
                             @_;

    @c ? @c : $c ? $c : ();
}
[download]

Makeshifts last the longest.