#! perl -slw
use strict;

sub recurse {
    print ">> '${ $_[ 0 ] }'";
    if( length ${ $_[ 0 ] } ) {
       #recurse( \            substr( ${ $_[ 0 ] }, 1, -1 )   );  #old
        recurse( \( my $boo = substr( ${ $_[ 0 ] }, 1, -1 ) ) );  #new
        ${ $_[ 0 ] } = " ( ${ $_[ 0 ] } ) ";
    }
    print "<< '${ $_[ 0 ] }'";
    return;
}

my $str = 'aaaaaaaaaa';
recurse \$str;

print $str;