Even better example:

#! perl -slw
use strict;

sub recurse {
    print ">> '${ $_[ 0 ] }'";
    (my $depth=$_[1])--;
    if ($depth) {
        recurse( \ substr( ${ $_[ 0 ] }, 1, -1 ), $depth );
    }        
    else {
        ${$_[0]} = "XX";
    }        
    print "<< '${ $_[ 0 ] }'";
    return;
}

my $str = 'abcdefghi';

recurse \$str, 3;
print $str;

recurse \$str, 2;
print $str;

__END__
>> 'abcdefghi'
>> 'bcdefgh'
>> 'cdefg'
<< 'XX'
<< 'bcdefgh'
<< 'abcdefghi'
abcdefghi
>> 'abcdefghi'
>> 'bcdefgh'
<< 'XXi'
<< 'aXXi'
aXXi
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