c:\@Work\Perl>perl -e "print prototype 'CORE::join'"
$@

##</code><code>##

c:\@Work\Perl>perl -wMstrict -MData::Dump -le
"my $output = 'something or other';
 ;;
 $_ = '   who knows what   ';
 my @outputlist = join s/(^\s+|\s+$)//g, $output;
 dd \@outputlist;
"
["something or other"]

##</code><code>##

c:\@Work\Perl>perl -wMstrict -MData::Dump -le
"my $output = 'something or other';
 ;;
 $_ = '   who knows what   ';
 my @outputlist = join 'anything at all', $output;
 dd \@outputlist;
"
["something or other"]

##</code><code>##

c:\@Work\Perl>perl -wMstrict -MData::Dump -le
"my $output = 'something or other';
 ;;
 $_ = '   who knows what   ';
 my @outputlist = join s/(^\s+|\s+$)//g, $output, 'hoo', 'ha';
 dd \@outputlist;
 print qq{default scalar after s///: '$_'};
"
["something or other2hoo2ha"]

default scalar after s///: 'who knows what'