use strict;
use warnings;
use Date::Calc "check_date";
print "Enter the date in yyyy-mm-dd format : "; # More "native" format.
while ( my $date = <STDIN> )
{
    last if $date =~ /^$/;
    my @date = $date =~ /(\d+)/g; # Allows any separator and mushy input.
    if ( eval { check_date( @date ) } )
    {
        print "You have entered valid data\n";
    }
    else
    {
        print "You have entered invalid data\n";
    }
    print "Enter the date in yyyy-mm-dd format : ";
}

__END__
Enter the date in yyyy-mm-dd format : ohai
You have entered invalid data
Enter the date in yyyy-mm-dd format : 2012-2-29
You have entered valid data
Enter the date in yyyy-mm-dd format : 2011-2-29
You have entered invalid data
Enter the date in yyyy-mm-dd format :