If you're retrieving your SQL DATETIME as a string, use something like Time::Piece's strptime or DateTime::Format::Strptime to convert it to a better representation, then use the provided methods to compare.

use DateTime ();
use DateTime::Format::Strptime ();
print DateTime::Format::Strptime
    ->new(pattern => '%Y-%m-%d %H:%M:%S.%3N')
    ->parse_datetime('2012-01-31 00:00:00.000')
    ->truncate(to=>'day')
    ->compare( DateTime->today );
# compare returns 0 if they're equal
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That might seem like a little overkill for what could be done with a simple string comparison of two dates in the same format, but can be very useful once time zones or other DateTime features get involved.