use strict;
use warnings;

use re 'eval';  # Should be scoped better.

sub rc($) {
    my $ofs = @- + shift;
    return substr($_, $-[$ofs], $+[$ofs] - $-[$ofs])
}

sub compile_pat { qr/$_[0]/ }

my @s_months = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
my $s_months = compile_pat join '|', @s_months;
local our
   %s_months = map { $s_months[$_] => $_+1 } 0..$#s_months;

my @pats = (
qr/ (\d{4})-(\d{2})-(\d{2})   (?{[ rc-3, rc-2,            rc-1 ]})/x,
qr/ (\d{2})($s_months)(\d{4}) (?{[ rc-1, $s_months{rc-2}, rc-3 ]})/x,
);

my $pat = compile_pat join '|', @pats;

for (qw(
    2009-12-01
    01Dec2009
    01-12-2009
)) {
    local our ($y,$m,$d);
    if (/$pat(?{ ($y,$m,$d) = @{$^R}  })/) {
        printf("%s => %04d-%02d-%02d\n", $_,$y,$m,$d);
    } else {
        printf("%s => [No match]\n", $_);
    }
}