As already explained, unshift is your answer, along with shift, push, and pop.

For a style hint, I'd just like to point the use of qw.

use Data::Dumper;

use strict;

my $form = {days => [qw(
    2011-02-26
    2011-02-25
    2011-02-24
    2011-02-23
    2011-02-22
    2011-02-21
    2011-02-20
)]};

unshift @{$form->{days}}, qw(2011-03-01 2011-02-28 2011-02-27);

print Dumper($form);
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