‘Gotcha’s 

my @month = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);

my @ymd = qw( 2001-08-31 1987-02-05 2003-03-12 1993-11-18 );

my @dmy = map {
    s/(\d\d\d\d)-(\d\d)-(\d\d)/$3 $month[$2 - 1] $1/;
    $_;
} @ymd;

say "$ymd[$_] => $dmy[$_]" foreach 0 .. 3;

31 Aug 2001 => 31 Aug 2001
05 Feb 1987 => 05 Feb 1987
12 Mar 2003 => 12 Mar 2003
18 Nov 1993 => 18 Nov 1993